feat(venue-crowd): toggleable crowd sound reactions on mood transitions

Rank up → the venue's cheer sample, rank down → boos, gated behind the
feedBack-venue-crowd-sfx setting (default off), rate-limited by the
existing transition hysteresis and suppressed during the intro. Sounds
ship per-venue in the pack manifest (sfx: {up, down}).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

lib/builtin_content.py

@@ -0,0 +1,378 @@
+"""Builtin content seeding: the calibration/diagnostic sloppaks and the starter library.
+
+Carved VERBATIM out of server.py (R3b) — with ONE deliberate signature change, and it is
+the whole reason this module is safe.
+
+━━━ WHY THE ROOT IS A PARAMETER ━━━
+
+server.py had `_feedBack_server_root()` = `Path(__file__).resolve().parent`. That is
+correct *in server.py*: the repo root in dev, resources/feedBack when bundled — the tree
+that actually holds docs/ and data/.
+
+Move that body here unchanged and it keeps working, silently, and returns `lib/`. There is
+no docs/diagnostics under lib/, so every seed would quietly find nothing and log "source
+missing" — a verbatim move whose meaning changed because `__file__` did. Nothing would
+fail; the starter library would just never appear.
+
+So this module CANNOT compute a root: it takes `server_root` as a parameter, and server.py
+— the only place that legitimately knows where it lives — passes it in. The trap is now
+structurally impossible rather than merely avoided. (_copy_builtin_packs already took the
+root this way; the two seed helpers now do too.)
+
+Everything else is byte-identical. `log` is this module's own logger under the same
+`feedBack.` hierarchy, and CONFIG_DIR is read late as `appstate.config_dir` — see appstate.py
+for why those reads must be late-bound (tests monkeypatch it).
+"""
+import logging
+import os
+import secrets
+import shutil
+import stat
+import tempfile
+from pathlib import Path
+
+import appstate
+from dlc_paths import _get_dlc_dir
+
+log = logging.getLogger("feedBack.builtin_content")
+
+
+BUILTIN_DIAGNOSTIC_SUBDIR = "diagnostics-builtin"
+
+
+BUILTIN_DIAGNOSTIC_SOURCES: list[tuple[str, str]] = [
+    (
+        "feedBack-diagnostic-basic-guitar.sloppak",
+        "docs/diagnostics/feedBack-diagnostic-basic-guitar.sloppak",
+    ),
+]
+
+
+def builtin_diagnostic_filename() -> str:
+    """Library filename (DLC-relative POSIX path) of the calibration sloppak —
+    the onboarding challenge target (spec 010)."""
+    return f"{BUILTIN_DIAGNOSTIC_SUBDIR}/{BUILTIN_DIAGNOSTIC_SOURCES[0][0]}"
+
+
+def _copy_builtin_packs(
+    root: Path,
+    dest_dir: Path,
+    sources: list[tuple[str, str]],
+    label: str,
+    update_existing: bool = True,
+) -> int:
+    """Symlink-safe, mtime-aware copy of bundled packs into ``dest_dir``.
+
+    ``sources`` is a list of ``(dest_name, rel_source)`` pairs; each source is
+    resolved under ``root`` (the repo root in dev, ``resources/feedBack`` when
+    bundled). A pack is copied when its destination is missing. Never deletes
+    user files; refuses to follow a symlinked seed directory or destination and
+    refuses to clobber a non-regular destination (any would let a copy escape
+    ``dest_dir`` or destroy user data). Logs and continues on error. ``label``
+    prefixes every log line.
+
+    ``update_existing`` controls what happens when a *regular* destination file
+    already exists: when True (diagnostic seed) a bundle copy newer than the
+    destination refreshes it; when False (one-time starter content) an existing
+    file is always left as-is so the user's copy is never overwritten.
+
+    Returns the number of ``sources`` that are present at their destination
+    afterwards (freshly seeded, refreshed, or already current) — so callers can
+    tell whether every pack made it. A skip (missing source, symlink/non-regular
+    refusal, copy error) does not count.
+    """
+    # Refuse a symlinked seed directory: mkdir(exist_ok=True) would accept it
+    # and copies would land at the link target, outside the DLC tree. The
+    # per-file symlink guard below cannot catch this.
+    if dest_dir.is_symlink():
+        log.warning("%s: %s is a symlink, skipping all seeding", label, dest_dir.name)
+        return 0
+    dest_dir.mkdir(parents=True, exist_ok=True)
+
+    # Pin the seed directory by an O_NOFOLLOW fd so a symlink swapped in for
+    # dest_dir *after* the check above cannot redirect the per-file stat /
+    # temp-create / replace outside the DLC tree (parent-directory TOCTOU).
+    # os.replace accepts dir_fd on POSIX even though it isn't listed in
+    # os.supports_dir_fd, so gate on os.rename (the reliable proxy); platforms
+    # without dir_fd/O_NOFOLLOW (e.g. Windows) fall back to path-based ops.
+    dir_fd = None
+    if (
+        hasattr(os, "O_NOFOLLOW")
+        and hasattr(os, "O_DIRECTORY")
+        and os.open in os.supports_dir_fd
+        and os.rename in os.supports_dir_fd
+    ):
+        try:
+            dir_fd = os.open(dest_dir, os.O_RDONLY | os.O_NOFOLLOW | os.O_DIRECTORY)
+        except OSError as exc:
+            log.warning("%s: cannot open seed dir %s: %s", label, dest_dir, exc)
+            return 0
+
+    try:
+        present = 0
+        for dest_name, rel_source in sources:
+            source = root / rel_source
+            if not source.is_file():
+                log.warning("%s: source missing, skipping %s (%s)", label, dest_name, source)
+                continue
+
+            # lstat the destination without following symlinks. Pinned by dir_fd
+            # this resolves within the real seed dir, immune to a parent swap.
+            try:
+                if dir_fd is not None:
+                    dstat = os.lstat(dest_name, dir_fd=dir_fd)
+                else:
+                    dstat = os.lstat(dest_dir / dest_name)
+                dest_exists = True
+                dest_islink = stat.S_ISLNK(dstat.st_mode)
+            except FileNotFoundError:
+                dest_exists = False
+                dest_islink = False
+            except OSError as exc:
+                log.warning("%s: cannot stat %s: %s", label, dest_name, exc)
+                continue
+
+            # Refuse to seed through a symlink at the destination name.
+            if dest_islink:
+                log.warning("%s: destination is a symlink, skipping %s", label, dest_name)
+                continue
+
+            # A non-regular destination (directory, fifo, …) the user placed
+            # there: never clobber it, and never count it as present — otherwise
+            # a one-time seed would mark itself done without a real pack on disk.
+            if dest_exists and not stat.S_ISREG(dstat.st_mode):
+                log.warning("%s: destination is not a regular file, skipping %s", label, dest_name)
+                continue
+
+            if dest_exists:
+                # A regular file is already there. One-time seeds (starter
+                # content) must never overwrite the user's copy; refreshing
+                # seeds (diagnostics) replace it only when the bundle is newer.
+                if not update_existing:
+                    log.info("%s: already present %s", label, dest_name)
+                    present += 1
+                    continue
+                try:
+                    src_mtime = source.stat().st_mtime
+                except OSError as exc:
+                    log.warning("%s: cannot stat source %s: %s", label, source, exc)
+                    continue
+                if src_mtime <= dstat.st_mtime:
+                    log.info("%s: already present %s", label, dest_name)
+                    present += 1
+                    continue
+                action = "updated"
+            else:
+                action = "seeded"
+
+            if _write_builtin_pack(source, dest_dir, dest_name, dir_fd):
+                present += 1
+                log.info("%s: %s %s -> %s", label, action, source.name, dest_name)
+            else:
+                log.warning("%s: failed to copy %s -> %s/%s", label, source, dest_dir.name, dest_name)
+
+        return present
+    finally:
+        if dir_fd is not None:
+            os.close(dir_fd)
+
+
+def _write_builtin_pack(
+    source: Path,
+    dest_dir: Path,
+    dest_name: str,
+    dir_fd: int | None,
+) -> bool:
+    """Atomically write ``source`` to ``dest_name`` inside ``dest_dir``.
+
+    Writes to a temp file then ``os.replace()``s onto the final name so a
+    symlink raced in at the destination is overwritten (rename semantics), not
+    followed, and a crash never leaves a half-written pack. When ``dir_fd`` is
+    given, every step is anchored to that fd (O_NOFOLLOW temp create + dir_fd
+    replace), closing the parent-directory TOCTOU; otherwise falls back to
+    path-based temp+replace. Returns True on success. Never raises.
+    """
+    # Unique per-attempt name (O_EXCL create) so a crash that orphans a temp
+    # can't permanently block later seeds via an EEXIST collision.
+    tmp_name = f".seed-{dest_name}.{os.getpid()}.{secrets.token_hex(4)}.tmp"
+    try:
+        src_stat = source.stat()
+    except OSError as exc:
+        log.debug("builtin pack: cannot stat source %s: %s", source, exc)
+        return False
+    if dir_fd is not None:
+        tmp_fd = None
+        try:
+            tmp_fd = os.open(
+                tmp_name,
+                os.O_CREAT | os.O_EXCL | os.O_WRONLY | os.O_NOFOLLOW,
+                0o644,
+                dir_fd=dir_fd,
+            )
+            with open(source, "rb") as sf, os.fdopen(tmp_fd, "wb") as tf:
+                tmp_fd = None  # fdopen now owns the descriptor
+                shutil.copyfileobj(sf, tf)
+            os.replace(tmp_name, dest_name, src_dir_fd=dir_fd, dst_dir_fd=dir_fd)
+            # Preserve the bundle mtime (copyfileobj doesn't) so the mtime-based
+            # refresh check matches the shutil.copy2 fallback path. Best-effort.
+            try:
+                os.utime(
+                    dest_name,
+                    ns=(src_stat.st_atime_ns, src_stat.st_mtime_ns),
+                    dir_fd=dir_fd,
+                    follow_symlinks=False,
+                )
+            except OSError as exc:
+                log.debug("builtin pack: could not set mtime on %s: %s", dest_name, exc)
+            return True
+        except OSError as exc:
+            log.debug("builtin pack write (dir_fd) failed for %s: %s", dest_name, exc)
+            if tmp_fd is not None:
+                try:
+                    os.close(tmp_fd)
+                except OSError:
+                    pass
+            try:
+                os.unlink(tmp_name, dir_fd=dir_fd)
+            except OSError:
+                pass
+            return False
+
+    tmp = None
+    try:
+        fd, tmp = tempfile.mkstemp(dir=dest_dir, prefix=".seed-", suffix=".tmp")
+        os.close(fd)
+        shutil.copy2(source, tmp)
+        os.replace(tmp, dest_dir / dest_name)
+        tmp = None
+        return True
+    except OSError as exc:
+        log.debug("builtin pack write failed for %s: %s", dest_name, exc)
+        return False
+    finally:
+        if tmp is not None:
+            try:
+                os.unlink(tmp)
+            except OSError:
+                pass
+
+
+def seed_builtin_diagnostic_sloppaks(server_root: Path, dlc: Path | None = None) -> None:
+    """Copy bundled diagnostic sloppaks into DLC before library scan.
+
+    Creates ``DLC_DIR/diagnostics-builtin/`` and copies each bundled sloppak
+    when the destination is missing or older than the repo/bundle source.
+    Never deletes user files or touches manually copied paths (e.g.
+    ``diagnostics-test/``). Re-seeds whenever the destination is missing so the
+    diagnostic target is always available. Logs and continues on errors.
+    """
+    try:
+        if dlc is None:
+            dlc = _get_dlc_dir()
+        if dlc is None:
+            log.debug("Builtin diagnostic seed: no DLC folder configured, skipping")
+            return
+        _copy_builtin_packs(
+            server_root,
+            dlc / BUILTIN_DIAGNOSTIC_SUBDIR,
+            BUILTIN_DIAGNOSTIC_SOURCES,
+            "Builtin diagnostic seed",
+        )
+    except Exception:
+        log.warning("Builtin diagnostic seed: unexpected error", exc_info=True)
+
+
+# Starter content: bundled songs copied into ``DLC_DIR/starter/`` exactly ONCE,
+# on first run, as a welcome library so a fresh install isn't empty. Unlike the
+# diagnostic seed this is one-time — guarded by a marker in CONFIG_DIR — so if
+# the user deletes the starter song it stays gone. ``starter/`` is NOT in the
+# library scan carve-out (unlike diagnostics-builtin/ / tutorials-builtin/), so
+# seeded packs surface as ordinary library songs.
+BUILTIN_STARTER_SUBDIR = "starter"
+
+
+BUILTIN_STARTER_SOURCES: list[tuple[str, str]] = [
+    (
+        "beethoven-fur_elise.feedpak",
+        "content/starter/beethoven-fur_elise.feedpak",
+    ),
+    (
+        "star_spangled_banner.feedpak",
+        "content/starter/star_spangled_banner.feedpak",
+    ),
+    (
+        "the_adicts-ode-to-joy_vst_cover.feedpak",
+        "content/starter/the_adicts-ode-to-joy_vst_cover.feedpak",
+    ),
+]
+
+
+STARTER_SEED_MARKER = ".starter-content-seeded"
+
+
+def seed_builtin_starter_content(server_root: Path, dlc: Path | None = None) -> None:
+    """Copy bundled starter songs into ``DLC_DIR/starter/`` exactly once.
+
+    Guarded by ``CONFIG_DIR/.starter-content-seeded``: the first run with a DLC
+    folder configured seeds the packs and writes the marker; subsequent runs are
+    no-ops, so a user who deletes the starter song does not get it back on the
+    next launch. Symlink-safe; never deletes user files. Logs, never raises.
+    """
+    try:
+        marker = appstate.config_dir / STARTER_SEED_MARKER
+        # Already seeded? The marker is a sentinel: any existing path there
+        # (regular file, or a symlink/dir a user deliberately planted to opt
+        # out) means "done" — lstat so we detect it without following a symlink.
+        # Worst case of a planted marker is simply no starter content, never a
+        # data write; the O_EXCL|O_NOFOLLOW create below refuses to write
+        # *through* a symlink regardless.
+        try:
+            os.lstat(marker)
+            return
+        except FileNotFoundError:
+            pass
+        except OSError as exc:
+            log.warning("Starter content seed: cannot stat marker %s: %s", marker, exc)
+            return
+        if dlc is None:
+            dlc = _get_dlc_dir()
+        if dlc is None:
+            # No DLC yet — leave the marker unwritten so we retry once a
+            # library folder is configured.
+            log.debug("Starter content seed: no DLC folder configured, skipping")
+            return
+        present = _copy_builtin_packs(
+            server_root,
+            dlc / BUILTIN_STARTER_SUBDIR,
+            BUILTIN_STARTER_SOURCES,
+            "Starter content seed",
+            update_existing=False,
+        )
+        # Only mark seeding complete once every starter pack is actually in
+        # place. If a source was missing or a copy failed, leave the marker
+        # unwritten so the next launch retries rather than permanently skipping.
+        if present < len(BUILTIN_STARTER_SOURCES):
+            log.info(
+                "Starter content seed: %d/%d packs present, will retry next launch",
+                present,
+                len(BUILTIN_STARTER_SOURCES),
+            )
+            return
+        # Record completion with an exclusive, no-follow create so a planted or
+        # raced symlink at the marker path can't redirect the write outside
+        # CONFIG_DIR. O_EXCL fails (EEXIST) on any existing path including a
+        # symlink, so we never write through one.
+        try:
+            appstate.config_dir.mkdir(parents=True, exist_ok=True)
+            flags = os.O_CREAT | os.O_EXCL | os.O_WRONLY | getattr(os, "O_NOFOLLOW", 0)
+            fd = os.open(marker, flags, 0o644)
+            try:
+                os.write(fd, b"1\n")
+            finally:
+                os.close(fd)
+        except FileExistsError:
+            pass  # already marked (or a non-regular path is squatting) — fine
+        except OSError as exc:
+            log.warning("Starter content seed: could not write marker %s: %s", marker, exc)
+    except Exception:
+        log.warning("Starter content seed: unexpected error", exc_info=True)

plugins/__init__.py

@@ -6,6 +6,7 @@ import json
 import logging
 import mimetypes
 import os
+import re
 import subprocess
 import sys
 import threading
@@ -2384,6 +2385,53 @@ def register_plugin_api(app: FastAPI):
                     return _plugin_file_response(request, script_file, "application/javascript")
         return Response("", status_code=404)
 
+    # ── Module-graph cache busting (#879) ────────────────────────────────
+    #
+    # ES modules are evaluated ONCE PER URL PER DOCUMENT. Re-inserting a
+    # <script type="module"> whose src the module map has already seen fires
+    # `load` but does NOT re-run the body. So re-loading a plugin — a rollback,
+    # and (see below) an upgrade too — silently kept the OLD module live while
+    # the loader recorded success: a no-op that reported it worked.
+    #
+    # Busting the ENTRY url does not help. A module plugin's screen.js is a
+    # one-line `import './src/main.js'`, and a relative specifier resolves
+    # against the base URL WITH THE QUERY DROPPED — so a ?v= token never reaches
+    # the graph. Driving a real browser through install -> upgrade -> rollback and
+    # counting evaluations of src/main.js gives ONE. The upgrade re-runs the shim
+    # at its new ?v= URL; the shim imports './src/main.js'; that resolves to the
+    # same URL; the module map returns the already-evaluated old module.
+    #
+    # So the token goes in the PATH: /api/plugins/<id>/g/<n>/screen.js. Every
+    # relative import inherits it at every depth — for free, with no
+    # import-specifier rewriting (which could never see `import(expr)` anyway).
+    #
+    # WHY A PATH REWRITE AND NOT TWO MIRRORED ROUTES. The token shifts the base
+    # URL, so EVERYTHING a module resolves relatively moves with it — not just
+    # imports. `new URL('../assets/worklet.js', import.meta.url)` from
+    # /api/plugins/x/g/1/src/main.js resolves to /api/plugins/x/g/1/assets/... .
+    # Mirroring only screen.js and src/ would fix imports and 404 every asset,
+    # worklet and wasm file the graph reaches — and would silently break again the
+    # next time someone adds a plugin route. Stripping the segment before routing
+    # makes every plugin route, present and future, work under the prefix.
+    #
+    # The token is opaque: it is never joined into a filesystem path (and is gone
+    # by the time any handler runs), so containment still rests entirely on the
+    # same safe_join the un-prefixed routes use.
+    _GEN_PREFIX = re.compile(r"^(/api/plugins/[^/]+)/g/[^/]+(/.+)$")
+
+    @app.middleware("http")
+    async def _strip_plugin_generation_prefix(request: Request, call_next):
+        m = _GEN_PREFIX.match(request.scope.get("path", ""))
+        if m:
+            # Starlette routes on scope["path"] alone. raw_path is deliberately left
+            # ALONE: it is informational, and re-encoding the rewritten str back to
+            # bytes would have to guess a codec — `.encode("latin-1")` raises
+            # UnicodeEncodeError on a perfectly valid plugin file like src/工具.js,
+            # 500ing a request the un-prefixed route serves fine. Leaving raw_path as
+            # the client actually sent it is also simply more truthful for logs.
+            request.scope["path"] = m.group(1) + m.group(2)
+        return await call_next(request)
+
     @app.get("/api/plugins/{plugin_id}/settings.html")
     def plugin_settings_html(plugin_id: str):
         with PLUGINS_LOCK:

plugins/highway_3d/screen.js

@@ -2418,6 +2418,13 @@
     let _venueSceneAssetsLoaded = false;
     let _venueSceneLoadFailed = false;
     const _venueTextureCache = new Map();
+    // Crowd video layers (career mode). venue-crowd.js owns the <video>
+    // elements and the crossfade timing; the renderer only maps them onto
+    // two planes in front of the static plate. _venueCrowdRev bumps on any
+    // element (re)assignment so update() knows to rebind textures.
+    const _venueCrowdVideos = [null, null];
+    let _venueCrowdMix = 0;
+    let _venueCrowdRev = 0;
 
     function _bgVenueMoodCoeffs(state) {
         const s = String(state || 'idle').toLowerCase();
@@ -2909,6 +2916,20 @@
     window.h3dVenueSceneSetMood = (state) => {
         _venueMoodState = String(state || 'idle').toLowerCase();
     };
+    // Crowd video layers (career mode) — see venue-crowd.js. Layer 0/1 are
+    // two coplanar backdrop planes; mix selects between them (0 → layer 0,
+    // 1 → layer 1) so the caller can crossfade loop videos.
+    window.h3dVenueBackdropSetVideo = (layer, videoEl) => {
+        const i = layer ? 1 : 0;
+        const el = videoEl || null;
+        if (_venueCrowdVideos[i] === el) return;
+        _venueCrowdVideos[i] = el;
+        _venueCrowdRev++;
+    };
+    window.h3dVenueBackdropSetMix = (mix) => {
+        const v = Number(mix);
+        _venueCrowdMix = Number.isFinite(v) ? Math.max(0, Math.min(1, v)) : 0;
+    };
     window.h3dVenueSceneSetInstrumentPov = (input) => {
         const next = _venueResolvePovFromInput(input);
         if (_venueInstrumentPov === next) return;
@@ -3371,6 +3392,40 @@
                     () => _venueMarkFailed('failed to load small-club bg plate'),
                 );
 
+                // Crowd video planes (career mode): two crossfading layers
+                // just in front of the static plate (which stays mounted as
+                // the no-pack / load-failure fallback). Textures bind lazily
+                // in update() when venue-crowd.js assigns video elements.
+                state.crowd = { layers: [], rev: -1 };
+                for (let i = 0; i < 2; i++) {
+                    const geo = new T.PlaneGeometry(1, 1);
+                    const mat = new T.MeshBasicMaterial({
+                        color: 0xffffff, transparent: true, opacity: 0,
+                        depthWrite: false, fog: false,
+                    });
+                    const mesh = new T.Mesh(geo, mat);
+                    mesh.visible = false;
+                    // Layer 1 sits nearest so three.js's back-to-front
+                    // transparent sort draws it after layer 0.
+                    const layer = {
+                        mesh, geo, mat, tex: null, videoEl: null,
+                        cam: settings.cam,
+                        distance: BG_BACKDROP_DISTANCE * (i === 0 ? 1.04 : 1.03),
+                        lastAspect: 0, lastVisibleHeight: 0,
+                    };
+                    layer.applyCoverCrop = function () {
+                        if (!layer.videoEl || !layer.tex) return;
+                        _bgCoverCrop(
+                            layer.tex,
+                            layer.videoEl.videoWidth || 0,
+                            layer.videoEl.videoHeight || 0,
+                            layer.cam.aspect,
+                        );
+                    };
+                    scene.add(mesh);
+                    state.crowd.layers.push(layer);
+                }
+
                 const hazeGeo = new T.PlaneGeometry(280 * K, 40 * K);
                 const hazeMat = new T.MeshBasicMaterial({
                     color: 0x101820, transparent: true, opacity: coeffs.haze,
@@ -3402,6 +3457,64 @@
                     s.haze.mat.opacity = (s.haze.baseOp || VENUE_HAZE_STEADY)
                         * (coeffs.haze / VENUE_HAZE_STEADY);
                 }
+                if (s.crowd) {
+                    // Rebind VideoTextures when venue-crowd.js (re)assigns
+                    // elements. VideoTexture samples the element every frame,
+                    // so a src change on the same element needs no rebind.
+                    if (s.crowd.rev !== _venueCrowdRev) {
+                        s.crowd.rev = _venueCrowdRev;
+                        s.crowd.layers.forEach((layer, i) => {
+                            const el = _venueCrowdVideos[i];
+                            if (layer.videoEl === el) return;
+                            if (layer.tex) { layer.mat.map = null; layer.tex.dispose(); layer.tex = null; }
+                            layer.videoEl = el;
+                            layer.lastAspect = 0; // force refit + recrop
+                            if (el) {
+                                const tex = new T.VideoTexture(el);
+                                tex.colorSpace = T.SRGBColorSpace;
+                                tex.wrapS = T.ClampToEdgeWrapping;
+                                tex.wrapT = T.ClampToEdgeWrapping;
+                                tex.minFilter = T.LinearFilter;
+                                tex.magFilter = T.LinearFilter;
+                                tex.generateMipmaps = false;
+                                layer.tex = tex;
+                                layer.mat.map = tex;
+                            }
+                            layer.mat.needsUpdate = true;
+                        });
+                    }
+                    const warm = coeffs.warmth;
+                    s.crowd.layers.forEach((layer, i) => {
+                        const el = layer.videoEl;
+                        // videoWidth === 0 until metadata lands — showing the
+                        // plane before that paints a black flash over the plate.
+                        const ready = !!el && el.videoWidth > 0;
+                        // venue-crowd.js swaps src on the same element (loop ↔
+                        // stinger); a new intrinsic size needs a fresh
+                        // cover-crop, which _bgFitBackdropPlane only reapplies
+                        // on camera aspect changes.
+                        if (ready && (layer.lastVidW !== el.videoWidth ||
+                                      layer.lastVidH !== el.videoHeight)) {
+                            layer.lastVidW = el.videoWidth;
+                            layer.lastVidH = el.videoHeight;
+                            layer.applyCoverCrop();
+                        }
+                        // Layer 0 (rear) stays fully opaque whenever any of the
+                        // fade involves it: two half-transparent layers would
+                        // let the static plate behind bleed through (~25% at
+                        // mid-fade). The crossfade is therefore layer 1 (front)
+                        // fading over an opaque layer 0 — in both directions.
+                        const opacity = i === 0
+                            ? (_venueCrowdMix < 0.999 ? 1 : 0)
+                            : _venueCrowdMix;
+                        layer.mat.opacity = opacity;
+                        layer.mesh.visible = ready && opacity > 0.01;
+                        if (layer.mesh.visible) {
+                            layer.mat.color.setRGB(warm, warm * 0.98, warm * 0.95);
+                            _bgFitBackdropPlane(layer);
+                        }
+                    });
+                }
             },
             teardown(s) {
                 if (!s) return;
@@ -3416,6 +3529,19 @@
                         p.mat.dispose?.();
                     }
                 }
+                // Crowd planes: this style owns the VideoTextures; the
+                // <video> elements belong to venue-crowd.js and survive.
+                if (s.crowd) {
+                    for (const layer of s.crowd.layers) {
+                        layer.mesh?.parent?.remove(layer.mesh);
+                        layer.geo?.dispose?.();
+                        if (layer.mat) {
+                            layer.mat.map = null;
+                            layer.mat.dispose?.();
+                        }
+                        layer.tex?.dispose?.();
+                    }
+                }
                 // Dispose the cached plate textures too — the module-level cache
                 // otherwise keeps every loaded POV plate GPU-resident for the
                 // page lifetime (steady VRAM growth across POV/arrangement swaps).

server.py

@@ -44,6 +44,7 @@ from dlc_paths import _get_dlc_dir, _resolve_dlc_path
 # `appstate.configure(...)` below publishes into the same namespace routers read.
 # Lives in lib/ because that is the one core dir every packaging path copies.
 import appstate
+import builtin_content
 # Extracted route modules. They import `appstate`, never `server` — one-way graph.
 from routers import audio_effects, artist_aliases, loops, playlists, ws_highway, chart, wanted, library_extras, shop, progression, profile, stats, version, diagnostics
 from routers import tunings as tunings_router
@@ -645,13 +646,6 @@ def _make_scan_executor():
     )
 
 
-_BUILTIN_DIAGNOSTIC_SUBDIR = "diagnostics-builtin"
-_BUILTIN_DIAGNOSTIC_SOURCES: list[tuple[str, str]] = [
-    (
-        "feedBack-diagnostic-basic-guitar.sloppak",
-        "docs/diagnostics/feedBack-diagnostic-basic-guitar.sloppak",
-    ),
-]
 
 
 def _feedBack_server_root() -> Path:
@@ -659,10 +653,6 @@ def _feedBack_server_root() -> Path:
     return Path(__file__).resolve().parent
 
 
-def _builtin_diagnostic_filename() -> str:
-    """Library filename (DLC-relative POSIX path) of the calibration sloppak —
-    the onboarding challenge target (spec 010)."""
-    return f"{_BUILTIN_DIAGNOSTIC_SUBDIR}/{_BUILTIN_DIAGNOSTIC_SOURCES[0][0]}"
 
 
 # Progression content (spec 010): bundled JSON under data/progression/ (paths,
@@ -694,329 +684,19 @@ def _get_progression_content() -> dict:
 # path is unchanged; routers call `appstate.get_progression_content()`.
 appstate.configure(
     get_progression_content=_get_progression_content,
-    builtin_diagnostic_filename=_builtin_diagnostic_filename,
+    builtin_diagnostic_filename=builtin_content.builtin_diagnostic_filename,
     tuning_providers=tuning_providers,
 )
 
 
-def _copy_builtin_packs(
-    root: Path,
-    dest_dir: Path,
-    sources: list[tuple[str, str]],
-    label: str,
-    update_existing: bool = True,
-) -> int:
-    """Symlink-safe, mtime-aware copy of bundled packs into ``dest_dir``.
-
-    ``sources`` is a list of ``(dest_name, rel_source)`` pairs; each source is
-    resolved under ``root`` (the repo root in dev, ``resources/feedBack`` when
-    bundled). A pack is copied when its destination is missing. Never deletes
-    user files; refuses to follow a symlinked seed directory or destination and
-    refuses to clobber a non-regular destination (any would let a copy escape
-    ``dest_dir`` or destroy user data). Logs and continues on error. ``label``
-    prefixes every log line.
-
-    ``update_existing`` controls what happens when a *regular* destination file
-    already exists: when True (diagnostic seed) a bundle copy newer than the
-    destination refreshes it; when False (one-time starter content) an existing
-    file is always left as-is so the user's copy is never overwritten.
-
-    Returns the number of ``sources`` that are present at their destination
-    afterwards (freshly seeded, refreshed, or already current) — so callers can
-    tell whether every pack made it. A skip (missing source, symlink/non-regular
-    refusal, copy error) does not count.
-    """
-    # Refuse a symlinked seed directory: mkdir(exist_ok=True) would accept it
-    # and copies would land at the link target, outside the DLC tree. The
-    # per-file symlink guard below cannot catch this.
-    if dest_dir.is_symlink():
-        log.warning("%s: %s is a symlink, skipping all seeding", label, dest_dir.name)
-        return 0
-    dest_dir.mkdir(parents=True, exist_ok=True)
-
-    # Pin the seed directory by an O_NOFOLLOW fd so a symlink swapped in for
-    # dest_dir *after* the check above cannot redirect the per-file stat /
-    # temp-create / replace outside the DLC tree (parent-directory TOCTOU).
-    # os.replace accepts dir_fd on POSIX even though it isn't listed in
-    # os.supports_dir_fd, so gate on os.rename (the reliable proxy); platforms
-    # without dir_fd/O_NOFOLLOW (e.g. Windows) fall back to path-based ops.
-    dir_fd = None
-    if (
-        hasattr(os, "O_NOFOLLOW")
-        and hasattr(os, "O_DIRECTORY")
-        and os.open in os.supports_dir_fd
-        and os.rename in os.supports_dir_fd
-    ):
-        try:
-            dir_fd = os.open(dest_dir, os.O_RDONLY | os.O_NOFOLLOW | os.O_DIRECTORY)
-        except OSError as exc:
-            log.warning("%s: cannot open seed dir %s: %s", label, dest_dir, exc)
-            return 0
-
-    try:
-        present = 0
-        for dest_name, rel_source in sources:
-            source = root / rel_source
-            if not source.is_file():
-                log.warning("%s: source missing, skipping %s (%s)", label, dest_name, source)
-                continue
-
-            # lstat the destination without following symlinks. Pinned by dir_fd
-            # this resolves within the real seed dir, immune to a parent swap.
-            try:
-                if dir_fd is not None:
-                    dstat = os.lstat(dest_name, dir_fd=dir_fd)
-                else:
-                    dstat = os.lstat(dest_dir / dest_name)
-                dest_exists = True
-                dest_islink = stat.S_ISLNK(dstat.st_mode)
-            except FileNotFoundError:
-                dest_exists = False
-                dest_islink = False
-            except OSError as exc:
-                log.warning("%s: cannot stat %s: %s", label, dest_name, exc)
-                continue
-
-            # Refuse to seed through a symlink at the destination name.
-            if dest_islink:
-                log.warning("%s: destination is a symlink, skipping %s", label, dest_name)
-                continue
-
-            # A non-regular destination (directory, fifo, …) the user placed
-            # there: never clobber it, and never count it as present — otherwise
-            # a one-time seed would mark itself done without a real pack on disk.
-            if dest_exists and not stat.S_ISREG(dstat.st_mode):
-                log.warning("%s: destination is not a regular file, skipping %s", label, dest_name)
-                continue
-
-            if dest_exists:
-                # A regular file is already there. One-time seeds (starter
-                # content) must never overwrite the user's copy; refreshing
-                # seeds (diagnostics) replace it only when the bundle is newer.
-                if not update_existing:
-                    log.info("%s: already present %s", label, dest_name)
-                    present += 1
-                    continue
-                try:
-                    src_mtime = source.stat().st_mtime
-                except OSError as exc:
-                    log.warning("%s: cannot stat source %s: %s", label, source, exc)
-                    continue
-                if src_mtime <= dstat.st_mtime:
-                    log.info("%s: already present %s", label, dest_name)
-                    present += 1
-                    continue
-                action = "updated"
-            else:
-                action = "seeded"
-
-            if _write_builtin_pack(source, dest_dir, dest_name, dir_fd):
-                present += 1
-                log.info("%s: %s %s -> %s", label, action, source.name, dest_name)
-            else:
-                log.warning("%s: failed to copy %s -> %s/%s", label, source, dest_dir.name, dest_name)
-
-        return present
-    finally:
-        if dir_fd is not None:
-            os.close(dir_fd)
 
 
-def _write_builtin_pack(
-    source: Path,
-    dest_dir: Path,
-    dest_name: str,
-    dir_fd: int | None,
-) -> bool:
-    """Atomically write ``source`` to ``dest_name`` inside ``dest_dir``.
-
-    Writes to a temp file then ``os.replace()``s onto the final name so a
-    symlink raced in at the destination is overwritten (rename semantics), not
-    followed, and a crash never leaves a half-written pack. When ``dir_fd`` is
-    given, every step is anchored to that fd (O_NOFOLLOW temp create + dir_fd
-    replace), closing the parent-directory TOCTOU; otherwise falls back to
-    path-based temp+replace. Returns True on success. Never raises.
-    """
-    # Unique per-attempt name (O_EXCL create) so a crash that orphans a temp
-    # can't permanently block later seeds via an EEXIST collision.
-    tmp_name = f".seed-{dest_name}.{os.getpid()}.{secrets.token_hex(4)}.tmp"
-    try:
-        src_stat = source.stat()
-    except OSError as exc:
-        log.debug("builtin pack: cannot stat source %s: %s", source, exc)
-        return False
-    if dir_fd is not None:
-        tmp_fd = None
-        try:
-            tmp_fd = os.open(
-                tmp_name,
-                os.O_CREAT | os.O_EXCL | os.O_WRONLY | os.O_NOFOLLOW,
-                0o644,
-                dir_fd=dir_fd,
-            )
-            with open(source, "rb") as sf, os.fdopen(tmp_fd, "wb") as tf:
-                tmp_fd = None  # fdopen now owns the descriptor
-                shutil.copyfileobj(sf, tf)
-            os.replace(tmp_name, dest_name, src_dir_fd=dir_fd, dst_dir_fd=dir_fd)
-            # Preserve the bundle mtime (copyfileobj doesn't) so the mtime-based
-            # refresh check matches the shutil.copy2 fallback path. Best-effort.
-            try:
-                os.utime(
-                    dest_name,
-                    ns=(src_stat.st_atime_ns, src_stat.st_mtime_ns),
-                    dir_fd=dir_fd,
-                    follow_symlinks=False,
-                )
-            except OSError as exc:
-                log.debug("builtin pack: could not set mtime on %s: %s", dest_name, exc)
-            return True
-        except OSError as exc:
-            log.debug("builtin pack write (dir_fd) failed for %s: %s", dest_name, exc)
-            if tmp_fd is not None:
-                try:
-                    os.close(tmp_fd)
-                except OSError:
-                    pass
-            try:
-                os.unlink(tmp_name, dir_fd=dir_fd)
-            except OSError:
-                pass
-            return False
-
-    tmp = None
-    try:
-        fd, tmp = tempfile.mkstemp(dir=dest_dir, prefix=".seed-", suffix=".tmp")
-        os.close(fd)
-        shutil.copy2(source, tmp)
-        os.replace(tmp, dest_dir / dest_name)
-        tmp = None
-        return True
-    except OSError as exc:
-        log.debug("builtin pack write failed for %s: %s", dest_name, exc)
-        return False
-    finally:
-        if tmp is not None:
-            try:
-                os.unlink(tmp)
-            except OSError:
-                pass
 
 
-def _seed_builtin_diagnostic_sloppaks(dlc: Path | None = None) -> None:
-    """Copy bundled diagnostic sloppaks into DLC before library scan.
-
-    Creates ``DLC_DIR/diagnostics-builtin/`` and copies each bundled sloppak
-    when the destination is missing or older than the repo/bundle source.
-    Never deletes user files or touches manually copied paths (e.g.
-    ``diagnostics-test/``). Re-seeds whenever the destination is missing so the
-    diagnostic target is always available. Logs and continues on errors.
-    """
-    try:
-        if dlc is None:
-            dlc = _get_dlc_dir()
-        if dlc is None:
-            log.debug("Builtin diagnostic seed: no DLC folder configured, skipping")
-            return
-        _copy_builtin_packs(
-            _feedBack_server_root(),
-            dlc / _BUILTIN_DIAGNOSTIC_SUBDIR,
-            _BUILTIN_DIAGNOSTIC_SOURCES,
-            "Builtin diagnostic seed",
-        )
-    except Exception:
-        log.warning("Builtin diagnostic seed: unexpected error", exc_info=True)
 
 
-# Starter content: bundled songs copied into ``DLC_DIR/starter/`` exactly ONCE,
-# on first run, as a welcome library so a fresh install isn't empty. Unlike the
-# diagnostic seed this is one-time — guarded by a marker in CONFIG_DIR — so if
-# the user deletes the starter song it stays gone. ``starter/`` is NOT in the
-# library scan carve-out (unlike diagnostics-builtin/ / tutorials-builtin/), so
-# seeded packs surface as ordinary library songs.
-_BUILTIN_STARTER_SUBDIR = "starter"
-_BUILTIN_STARTER_SOURCES: list[tuple[str, str]] = [
-    (
-        "beethoven-fur_elise.feedpak",
-        "content/starter/beethoven-fur_elise.feedpak",
-    ),
-    (
-        "star_spangled_banner.feedpak",
-        "content/starter/star_spangled_banner.feedpak",
-    ),
-    (
-        "the_adicts-ode-to-joy_vst_cover.feedpak",
-        "content/starter/the_adicts-ode-to-joy_vst_cover.feedpak",
-    ),
-]
-_STARTER_SEED_MARKER = ".starter-content-seeded"
 
 
-def _seed_builtin_starter_content(dlc: Path | None = None) -> None:
-    """Copy bundled starter songs into ``DLC_DIR/starter/`` exactly once.
-
-    Guarded by ``CONFIG_DIR/.starter-content-seeded``: the first run with a DLC
-    folder configured seeds the packs and writes the marker; subsequent runs are
-    no-ops, so a user who deletes the starter song does not get it back on the
-    next launch. Symlink-safe; never deletes user files. Logs, never raises.
-    """
-    try:
-        marker = CONFIG_DIR / _STARTER_SEED_MARKER
-        # Already seeded? The marker is a sentinel: any existing path there
-        # (regular file, or a symlink/dir a user deliberately planted to opt
-        # out) means "done" — lstat so we detect it without following a symlink.
-        # Worst case of a planted marker is simply no starter content, never a
-        # data write; the O_EXCL|O_NOFOLLOW create below refuses to write
-        # *through* a symlink regardless.
-        try:
-            os.lstat(marker)
-            return
-        except FileNotFoundError:
-            pass
-        except OSError as exc:
-            log.warning("Starter content seed: cannot stat marker %s: %s", marker, exc)
-            return
-        if dlc is None:
-            dlc = _get_dlc_dir()
-        if dlc is None:
-            # No DLC yet — leave the marker unwritten so we retry once a
-            # library folder is configured.
-            log.debug("Starter content seed: no DLC folder configured, skipping")
-            return
-        present = _copy_builtin_packs(
-            _feedBack_server_root(),
-            dlc / _BUILTIN_STARTER_SUBDIR,
-            _BUILTIN_STARTER_SOURCES,
-            "Starter content seed",
-            update_existing=False,
-        )
-        # Only mark seeding complete once every starter pack is actually in
-        # place. If a source was missing or a copy failed, leave the marker
-        # unwritten so the next launch retries rather than permanently skipping.
-        if present < len(_BUILTIN_STARTER_SOURCES):
-            log.info(
-                "Starter content seed: %d/%d packs present, will retry next launch",
-                present,
-                len(_BUILTIN_STARTER_SOURCES),
-            )
-            return
-        # Record completion with an exclusive, no-follow create so a planted or
-        # raced symlink at the marker path can't redirect the write outside
-        # CONFIG_DIR. O_EXCL fails (EEXIST) on any existing path including a
-        # symlink, so we never write through one.
-        try:
-            CONFIG_DIR.mkdir(parents=True, exist_ok=True)
-            flags = os.O_CREAT | os.O_EXCL | os.O_WRONLY | getattr(os, "O_NOFOLLOW", 0)
-            fd = os.open(marker, flags, 0o644)
-            try:
-                os.write(fd, b"1\n")
-            finally:
-                os.close(fd)
-        except FileExistsError:
-            pass  # already marked (or a non-regular path is squatting) — fine
-        except OSError as exc:
-            log.warning("Starter content seed: could not write marker %s: %s", marker, exc)
-    except Exception:
-        log.warning("Starter content seed: unexpected error", exc_info=True)
 
 
 def _background_scan():
@@ -1038,8 +718,8 @@ def _background_scan():
         log.warning("Scan: no DLC folder configured")
         return
 
-    _seed_builtin_diagnostic_sloppaks(dlc)
-    _seed_builtin_starter_content(dlc)
+    builtin_content.seed_builtin_diagnostic_sloppaks(_feedBack_server_root(), dlc)
+    builtin_content.seed_builtin_starter_content(_feedBack_server_root(), dlc)
 
     # Listing can fail on macOS without Full Disk Access, or on Docker if the
     # path isn't shared. Report the failure explicitly rather than silently

static/js/audio-el.js

@@ -0,0 +1,17 @@
+// The one <audio> element the whole app plays through.
+//
+// This exists so that code carved out of app.js can reach the player without
+// importing app.js back — which would close a cycle and fail the import-x/no-cycle
+// gate. It is the same handle app.js has always held (`document.getElementById`
+// on the element in the shell), just given a home of its own.
+//
+// It is deliberately a `const`, and it is never reassigned anywhere in core — so a
+// read-only import binding is exactly right, and no state container is needed.
+// (Contrast the reassigned scalars — isPlaying, _avOffsetMs, … — which cannot be
+// shared this way, because an imported binding cannot be written to.)
+//
+// Module scripts evaluate after the HTML is parsed, so the element is already in
+// the document by the time this runs. app.js is loaded as <script type="module">,
+// and its imports evaluate before its body — the same point at which app.js used
+// to run this exact lookup itself.
+export const audio = document.getElementById('audio');

static/js/count-in.js

@@ -0,0 +1,389 @@
+// Count-in — the 1-2-3-4 click before playback, plus the song-credits overlay that
+// shares its lifecycle and timers.
+//
+// The third slice out of app.js's strongly-connected core, and the first that had to
+// WRITE shared state rather than just read it. It starts and stops playback, so it sets
+// `isPlaying` and `lastAudioTime`. An imported binding is read-only — `isPlaying = true`
+// throws — which is exactly why those two scalars were lifted onto the container in
+// ./player-state.js. Every earlier slice only READ what it shared, so a getter hook
+// sufficed; this one could not.
+//
+// It imports the loop module directly (setLoop / loopA / loopB — a count-in that starts
+// inside an A-B loop must begin at A). Nothing imports count-in back: app.js and
+// section-practice both reach it through the host seam, so the graph stays acyclic.
+//
+// app.js's autoplay path used to reach IN and set the credits timers itself. It cannot
+// now, and it should not have to — so the module exports the OPERATIONS instead
+// (armCreditsHideOnPlay, scheduleCreditsHide, holdCreditsThen, isCountingIn) and owns
+// its own timer invariants. Same reason section-practice grew resetSelection().
+//
+// See ./host.js: reading an unwired hook THROWS, and tests/js/host_contract.test.js
+// fails CI if the hooks used here and the hooks app.js wires ever drift apart.
+import { audio } from './audio-el.js';
+import { _audioSeek, _songEventPayload, jucePlayer, setPlayButtonState, togglePlay } from './transport.js';
+import { loopA, loopB, setLoop } from './loops.js';
+import { S } from './player-state.js';
+
+// ── Count-in click sound (Web Audio API) ────────────────────────────────
+let _audioCtx = null;
+export function playClick(high = false) {
+    if (!_audioCtx) _audioCtx = new (window.AudioContext || window.webkitAudioContext)();
+    const osc = _audioCtx.createOscillator();
+    const gain = _audioCtx.createGain();
+    osc.connect(gain);
+    gain.connect(_audioCtx.destination);
+    osc.frequency.value = high ? 1200 : 800;
+    osc.type = 'sine';
+    gain.gain.setValueAtTime(0.5, _audioCtx.currentTime);
+    gain.gain.exponentialRampToValueAtTime(0.001, _audioCtx.currentTime + 0.08);
+    osc.start(_audioCtx.currentTime);
+    osc.stop(_audioCtx.currentTime + 0.08);
+}
+
+let _countingIn = false;
+let _countOverlay = null;
+// Generation token so teardown can cancel an in-progress count-in. Each
+// startCountIn() captures the gen at entry; rewindStep, the loop-wrap
+// then-callback, and beginCount's tick all bail when their captured gen
+// no longer matches. Bumped by _cancelCountIn().
+let _countInGen = 0;
+let _countInTimer = null;
+let _countInRaf = 0;
+// Feedpak credits overlay (manifest `authors:`, spec §5.4): shown on the
+// highway when a song is loaded, alongside the count-in. Torn down together
+// with the count-in via _cancelCountIn().
+let _creditsOverlay = null;
+let _creditsTimer = null;
+let _creditsHideOnPlay = null;
+let _creditsMaxTimer = null;
+const _CREDITS_HOLD_MS = 3000;
+// Backstop: the overlay's primary dismiss is song:play, but playback can fail
+// to start without emitting it (HTML5 autoplay rejection, JUCE start failure,
+// a count-in handoff that never plays). This hard cap guarantees the credits
+// never linger over the highway. Generous enough to outlast a normal count-in.
+const _CREDITS_MAX_MS = 12000;
+export function _cancelCountIn() {
+    _countInGen++;
+    _countingIn = false;
+    hideCountOverlay();
+    // The credits overlay rides the count-in lifecycle (and its no-count-in
+    // hold timer), so a teardown — leaving the player, loading another song —
+    // must clear it too, or it lingers on the next screen.
+    hideSongCreditsOverlay();
+    if (_countInTimer) { clearTimeout(_countInTimer); _countInTimer = null; }
+    if (_countInRaf) { cancelAnimationFrame(_countInRaf); _countInRaf = 0; }
+}
+
+export function showCountOverlay(n) {
+    if (!_countOverlay) {
+        _countOverlay = document.createElement('div');
+        _countOverlay.className = 'fixed inset-0 z-[100] flex items-center justify-center pointer-events-none';
+        document.body.appendChild(_countOverlay);
+    }
+    _countOverlay.innerHTML = `<span class="text-9xl font-black text-white/30">${n}</span>`;
+}
+
+export function hideCountOverlay() {
+    if (_countOverlay) { _countOverlay.remove(); _countOverlay = null; }
+}
+
+// Map a feedpak author `role` to a friendly "<verb> by" credit line. The
+// recommended vocabulary is from feedpak spec §5.4; unknown roles are
+// title-cased ("foo" → "Foo by"); a missing role shows the bare name.
+const _CREDIT_ROLE_VERBS = {
+    charter: 'Charted by',
+    transcriber: 'Transcribed by',
+    arranger: 'Arranged by',
+    editor: 'Edited by',
+    mixer: 'Mixed by',
+    engineer: 'Engineered by',
+    proofreader: 'Proofread by',
+};
+
+function _creditLineLabel(role) {
+    if (!role) return '';
+    const key = String(role).trim().toLowerCase();
+    if (_CREDIT_ROLE_VERBS[key]) return _CREDIT_ROLE_VERBS[key];
+    return key.charAt(0).toUpperCase() + key.slice(1) + ' by';
+}
+
+// Show the feedpak contributor credits over the highway. `authors` is the
+// sanitized [{name, role}] list from window.feedBack.currentSong.authors.
+// Anchored to the lower third (bottom-center) so it never collides with the
+// vertically-centered count-in number, and pointer-events-none so it never
+// intercepts clicks. No-op when there are no contributors to show.
+export function showSongCreditsOverlay(authors) {
+    if (!Array.isArray(authors) || authors.length === 0) return;
+    if (!_creditsOverlay) {
+        _creditsOverlay = document.createElement('div');
+        _creditsOverlay.className = 'song-credits-overlay';
+        document.body.appendChild(_creditsOverlay);
+    }
+    // Build via DOM + textContent — author names are untrusted pack data and
+    // must never be interpolated as HTML.
+    _creditsOverlay.replaceChildren();
+    const card = document.createElement('div');
+    card.className = 'song-credits-card';
+
+    const eyebrow = document.createElement('div');
+    eyebrow.className = 'song-credits-eyebrow';
+    eyebrow.textContent = 'Credits';
+    card.appendChild(eyebrow);
+
+    const title = (window.feedBack && window.feedBack.currentSong
+        && window.feedBack.currentSong.title) || '';
+    if (title) {
+        const heading = document.createElement('div');
+        heading.className = 'song-credits-heading';
+        heading.textContent = title;
+        card.appendChild(heading);
+    }
+
+    for (const a of authors) {
+        if (!a || !a.name) continue;
+        const row = document.createElement('div');
+        row.className = 'song-credits-line';
+        const label = _creditLineLabel(a.role);
+        if (label) {
+            const lab = document.createElement('span');
+            lab.className = 'song-credits-role';
+            lab.textContent = label + ' ';
+            row.appendChild(lab);
+        }
+        const nm = document.createElement('span');
+        nm.className = 'song-credits-name';
+        nm.textContent = a.name;
+        row.appendChild(nm);
+        card.appendChild(row);
+    }
+    _creditsOverlay.appendChild(card);
+    // Arm the backstop so the overlay self-clears even if playback never starts
+    // / never emits song:play. song:play (or any teardown) clears it earlier.
+    if (_creditsMaxTimer) clearTimeout(_creditsMaxTimer);
+    _creditsMaxTimer = setTimeout(hideSongCreditsOverlay, _CREDITS_MAX_MS);
+}
+
+export function hideSongCreditsOverlay() {
+    if (_creditsTimer) { clearTimeout(_creditsTimer); _creditsTimer = null; }
+    if (_creditsMaxTimer) { clearTimeout(_creditsMaxTimer); _creditsMaxTimer = null; }
+    if (_creditsHideOnPlay) {
+        window.feedBack.off('song:play', _creditsHideOnPlay);
+        _creditsHideOnPlay = null;
+    }
+    if (_creditsOverlay) { _creditsOverlay.remove(); _creditsOverlay = null; }
+}
+
+export async function startCountIn(opts = {}) {
+    if (_countingIn) return;
+    _countingIn = true;
+    // Snapshot the current gen so every delayed callback (rewind frames,
+    // post-seek then, count-in ticks, post-count play) can bail if a
+    // teardown bumped the gen mid-flight via _cancelCountIn().
+    const gen = _countInGen;
+    const immediate = !!opts.immediate;
+    if (window._juceMode) {
+        await jucePlayer.pause().catch((err) => console.error('[app] jucePlayer.pause error in count-in:', err));
+    } else {
+        audio.pause();
+    }
+    if (gen !== _countInGen) return; // teardown during pause
+
+    // Section-practice entry: already at loop A after setLoop(); skip the
+    // B→A rewind animation used on loop wrap and go straight to clicks.
+    if (immediate) {
+        if (loopA === null || loopB === null) {
+            _countingIn = false;
+            return;
+        }
+        S.lastAudioTime = loopA;
+        highway.setTime(loopA);
+        if (window.feedBack) {
+            window.feedBack.emit('loop:restart', { loopA, loopB, time: loopA });
+        }
+        beginCount();
+        return;
+    }
+
+    // Rewind animation: sweep highway time from B to A
+    const rewindDuration = 400; // ms
+    const rewindStart = performance.now();
+    const fromTime = loopB;
+    const toTime = loopA;
+
+    function rewindStep(now) {
+        if (gen !== _countInGen) return; // teardown mid-rewind
+        const elapsed = now - rewindStart;
+        const t = Math.min(elapsed / rewindDuration, 1);
+        // Ease out quad
+        const eased = 1 - (1 - t) * (1 - t);
+        const currentT = fromTime + (toTime - fromTime) * eased;
+        highway.setTime(currentT);
+        if (t < 1) {
+            _countInRaf = requestAnimationFrame(rewindStep);
+        } else {
+            _countInRaf = 0;
+            // Rewind done — set final position and start count.
+            // Await the JUCE seek so the engine has repositioned before
+            // we start the click track (HTML5 path is synchronous).
+            _audioSeek(loopA, 'loop-wrap').then((r) => {
+                if (gen !== _countInGen) return; // teardown during seek
+                // Abort the loop restart in two cases:
+                //   1. Cancelled (player torn down): don't beginCount on a
+                //      new session.
+                //   2. Off-target landing (JUCE rollback / clamp far from
+                //      loopA): proceeding would emit loop:restart and start
+                //      a count-in from the wrong position. Audio is at
+                //      r.from / r.to, which is not where the loop wants to
+                //      resume — better to drop this iteration than play out
+                //      of sync.
+                // 50 ms tolerance: well within JUCE's normal seek precision
+                // but tight enough to catch a real rollback or no-op.
+                if (!r.completed || Math.abs(r.to - loopA) > 0.05) {
+                    // startCountIn paused audio at entry but left isPlaying
+                    // alone — beginCount would have set it on resume. On
+                    // abort, sync the transport: audio is paused, so
+                    // isPlaying must reflect that and the button + plugin
+                    // host must agree.
+                    _countingIn = false;
+                    if (S.isPlaying) {
+                        S.isPlaying = false;
+                        setPlayButtonState(false);
+                        if (window.feedBack) {
+                            window.feedBack.isPlaying = false;
+                            window.feedBack.emit('song:pause', _songEventPayload());
+                        }
+                    }
+                    return;
+                }
+                // Use the verified post-seek clock for the chart so audio
+                // and chart stay in sync if JUCE clamped to slightly
+                // before/after loopA. The loop:restart event keeps `time:
+                // loopA` because subscribers treat that as the semantic
+                // marker for "new iteration starts at A", not the actual
+                // audio position.
+                S.lastAudioTime = r.to;
+                highway.setTime(r.to);
+                window.feedBack.emit('loop:restart', { loopA, loopB, time: loopA });
+                beginCount();
+            });
+        }
+    }
+    _countInRaf = requestAnimationFrame(rewindStep);
+
+    function beginCount() {
+        const bpm = highway.getBPM(loopA);
+        const beatInterval = 60 / bpm;
+        let count = 0;
+
+        function tick() {
+            if (gen !== _countInGen) return; // teardown mid-count
+            count++;
+            if (count > 4) {
+                hideCountOverlay();
+                _countingIn = false;
+                if (window._juceMode) {
+                    jucePlayer.play().then((started) => {
+                        if (gen !== _countInGen) return; // teardown during play start
+                        if (!started) return;
+                        S.isPlaying = true;
+                        setPlayButtonState(true);
+                        window.feedBack.isPlaying = true;
+                        const payload = _songEventPayload();
+                        window.feedBack.emit('song:play', payload);
+                        window.feedBack.emit('song:resume', payload);
+                    }).catch((err) => console.error('[app] jucePlayer.play error:', err));
+                } else {
+                    audio.play().then(() => {
+                        if (gen !== _countInGen) return;
+                        S.isPlaying = true;
+                        setPlayButtonState(true);
+                    }).catch((err) => {
+                        if (gen !== _countInGen) return;
+                        // An engine reroute's deliberate pause aborts this play()
+                        // while playback continues on JUCE — don't reset the
+                        // button (mirrors the togglePlay guard).
+                        if (window._juceRerouteInProgress) return;
+                        // Same rationale as togglePlay: don't claim playback
+                        // started if the Promise rejected.
+                        console.error('[app] audio.play() rejected after count-in:', err);
+                        S.isPlaying = false;
+                        setPlayButtonState(false);
+                    });
+                }
+                return;
+            }
+            showCountOverlay(count);
+            playClick(count === 1);
+            _countInTimer = setTimeout(tick, beatInterval * 1000);
+        }
+        _countInTimer = setTimeout(tick, 500);
+    }
+}
+
+// Start-of-song count-in: a 4-beat click before playback begins, gated by the
+// "Countdown before song" setting (Gameplay tab). Mirrors the loop count-in's
+// overlay + click + gen-token cancellation, but counts from the song's current
+// position (0 at song start) with no loop A/B rewind. startCountIn() is loop-
+// coupled (early-returns when loopA/loopB are null), so this is a sibling
+// rather than an overload. Hands off to togglePlay() once the count completes.
+export async function startSongCountIn() {
+    if (_countingIn) return;
+    _countingIn = true;
+    // Snapshot the gen so a teardown (showScreen/playSong calls _cancelCountIn)
+    // bumps it and every delayed callback below bails.
+    const gen = _countInGen;
+    if (window._juceMode) {
+        await jucePlayer.pause().catch((err) => console.error('[app] jucePlayer.pause error in song count-in:', err));
+    } else {
+        audio.pause();
+    }
+    if (gen !== _countInGen) return; // teardown during pause
+    const startT = S.lastAudioTime || 0;
+    let bpm = highway.getBPM(startT);
+    // Pre-chart / malformed-tempo fallback: 4 beats at 120 BPM (500 ms each).
+    if (!Number.isFinite(bpm) || bpm <= 0) bpm = 120;
+    const beatInterval = 60 / bpm;
+    let count = 0;
+    function tick() {
+        if (gen !== _countInGen) return; // teardown mid-count
+        count++;
+        if (count > 4) {
+            hideCountOverlay();
+            _countingIn = false;
+            // Hand off to the normal play path — togglePlay() flips isPlaying,
+            // updates the button, and emits song:play/resume for plugins.
+            Promise.resolve(togglePlay()).catch((err) => console.warn('[app] play after count-in failed:', err));
+            return;
+        }
+        showCountOverlay(count);
+        playClick(count === 1);
+        _countInTimer = setTimeout(tick, beatInterval * 1000);
+    }
+    // First beat after a short lead-in, matching the loop count-in's 500 ms.
+    _countInTimer = setTimeout(tick, 500);
+}
+
+// ── Operations app.js's autoplay path used to perform by reaching in ────────
+// It used to assign _creditsTimer / _creditsHideOnPlay directly. Imported bindings are
+// read-only, and the module should own its own timer invariants anyway.
+
+/** Is a count-in running? app.js's timeupdate handler suppresses highway sync during one. */
+export function isCountingIn() {
+    return _countingIn;
+}
+
+/** Dismiss the credits the moment real playback begins. Fires once. */
+export function armCreditsHideOnPlay() {
+    _creditsHideOnPlay = () => { _creditsHideOnPlay = null; hideSongCreditsOverlay(); };
+    window.feedBack.on('song:play', _creditsHideOnPlay, { once: true });
+}
+
+/** Let the credits dwell, then clear them. Used when autoplay-exit is disabled. */
+export function scheduleCreditsHide() {
+    _creditsTimer = setTimeout(hideSongCreditsOverlay, _CREDITS_HOLD_MS);
+}
+
+/** Let the credits dwell, then run `then` (the autoplay start). */
+export function holdCreditsThen(then) {
+    _creditsTimer = setTimeout(() => { _creditsTimer = null; then(); }, _CREDITS_HOLD_MS);
+}

static/js/format.js

@@ -0,0 +1,17 @@
+// Display formatters. A LEAF module: imports nothing.
+//
+// WHY THIS EXISTS FOR ONE FUNCTION. formatTime was a HOST HOOK — loops.js and
+// section-practice.js both reached back through the seam for it. It was also, by pure
+// accident of who calls it, inside the dependency closure of the library carve. Leaving
+// it there would have made loops.js and section-practice.js import the LIBRARY to format
+// a timestamp, which is nonsense, and a cycle waiting to happen.
+//
+// A hook is a cycle you agreed to live with. This one has a real owner — it just isn't
+// app.js, and it certainly isn't the library. Give it a home of its own and both
+// consumers import it directly.
+//
+// It is a leaf on purpose. Anything else that turns out to be a shared pure formatter
+// belongs here too; nothing does yet, so nothing else is here.
+
+/** Seconds -> `M:SS`. */
+export function formatTime(s) { return `${Math.floor(s / 60)}:${String(Math.floor(s % 60)).padStart(2, '0')}`; }

static/js/host.js

@@ -0,0 +1,99 @@
+// The host seam — how a carved-out module calls back into app.js.
+//
+// WHY THIS EXISTS. What is left in app.js is not a tree, it is a cycle: seeding a
+// dependency closure from count-in, from loops, from section-practice, or from the
+// JUCE seek shim all return the SAME 178-function set, and setLoop() and
+// practiceSection() call each other directly. So a module carved out of that
+// component will always need to call back into app.js — and it cannot `import`
+// app.js to do it, because app.js imports the module, and that closes a cycle the
+// import-x/no-cycle gate (rightly) rejects.
+//
+// So app.js hands its functions DOWN, once, at boot: `configureHost({ playSong, … })`.
+//
+// ─── THE FAILURE MODE THIS IS BUILT TO PREVENT ───────────────────────────────
+//
+// The obvious way to write this is a plain object with no-op defaults. That is a
+// TRAP, and we walked into it once already: the plugin loader's host seam defaulted
+// `populateVizPicker` to `() => {}`, which means that if the wiring call in app.js
+// is ever dropped, renamed, or drifts, the loader keeps running, the viz picker
+// silently stops refreshing, and NOTHING — no test, no boot check, no bot — says a
+// word. A feature just quietly stops existing.
+//
+// Two layers stop that here, and the second is the one that actually closes it:
+//
+//   1. RUNTIME — reading an unwired hook THROWS. There are no defaults and no
+//      stubs. `host.playSong` either is the real function or it is a loud error.
+//      An unwired hook cannot degrade into a no-op, because there is nothing for
+//      it to degrade INTO.
+//
+//   2. STATIC — tests/js/host_contract.test.js asserts that the set of hooks the
+//      modules USE is exactly the set app.js WIRES. This is the important one:
+//      layer 1 only fires if the broken path actually executes, and the whole
+//      danger of this seam is paths that don't run in a smoke test. The static
+//      check catches a drifted or misspelled hook in CI, on a path nobody ran.
+//
+// Consequence for anyone adding a hook: add it to the configureHost({…}) call in
+// app.js *and* use it as `host.<name>`. The contract test fails on either alone —
+// deliberately. A hook wired but never used is dead weight; a hook used but never
+// wired is a bug that would otherwise hide.
+
+const _hooks = Object.create(null);
+let _configured = false;
+
+/**
+ * Called ONCE by app.js at boot, before any carved module runs. Every value must
+ * be a function — a hook that is accidentally `undefined` (a typo, a renamed
+ * export, a dropped line) fails HERE, at startup, rather than silently much later.
+ */
+export function configureHost(hooks) {
+    if (_configured) {
+        throw new Error('[host] configureHost() called twice — it must be wired exactly once, at boot.');
+    }
+    const bad = Object.entries(hooks || {})
+        .filter(([, v]) => typeof v !== 'function')
+        .map(([k]) => k);
+    if (bad.length) {
+        throw new Error(
+            `[host] these hooks are not functions: ${bad.join(', ')}. `
+            + 'A hook is usually undefined because it was renamed or its line was dropped.',
+        );
+    }
+    Object.assign(_hooks, hooks);
+    _configured = true;
+}
+
+/**
+ * The seam itself. Reading a hook that was never wired THROWS — it never returns
+ * undefined and never returns a silent no-op. See the note at the top: a no-op
+ * default is precisely the bug this module exists to make impossible.
+ */
+export const host = new Proxy(Object.create(null), {
+    get(_target, name) {
+        if (typeof name === 'symbol') return undefined;   // let JS probe it freely
+        if (!_configured) {
+            throw new Error(
+                `[host] host.${name} was read before configureHost() ran. `
+                + 'app.js must call configureHost() at boot, before any carved module executes.',
+            );
+        }
+        const fn = _hooks[name];
+        if (typeof fn !== 'function') {
+            throw new Error(
+                `[host] host.${name} is not wired. Add it to the configureHost({ … }) `
+                + 'call in app.js. (tests/js/host_contract.test.js should have caught this in CI.)',
+            );
+        }
+        return fn;
+    },
+    // Keep the object honest for anything that introspects it.
+    has(_target, name) { return name in _hooks; },
+    ownKeys() { return Object.keys(_hooks); },
+    getOwnPropertyDescriptor(_target, name) {
+        return name in _hooks
+            ? { value: _hooks[name], enumerable: true, configurable: true, writable: false }
+            : undefined;
+    },
+    set(_target, name) {
+        throw new Error(`[host] host.${String(name)} is read-only — hooks are wired only via configureHost().`);
+    },
+});

static/js/juce-audio.js

@@ -0,0 +1,994 @@
+// The desktop (JUCE) audio integration — three self-installing shims.
+//
+// The largest single slice out of app.js's core: 938 lines, ~12% of what was left.
+//
+//   _installJuceEngineRoutingWatcher  routes a song to the JUCE engine or HTML5 as the
+//                                     desktop output device enters/leaves exclusive/ASIO
+//   _installRendererBusFeeder         feeds the highway renderer bus from whichever
+//                                     transport is actually running
+//   _installJuceAudioElementShim      patches audio.play/pause so the rest of the app
+//                                     can keep talking to the <audio> element while JUCE
+//                                     owns the transport
+//
+// They EXPORT NOTHING. All three are IIFEs that publish through `window.*`
+// (_juceMode, _reevaluateJuceRouting, _reevaluateRendererBus, …) — which is why app.js
+// only needs a side-effect import for two of them, plus _resetJuceAudioShimChain.
+//
+// ORDERING, CHECKED: importing this module runs the IIFEs EARLIER than before —
+// imports evaluate ahead of app.js's body, and therefore ahead of configureHost().
+// That is safe because none of them touches a hook at execution depth: they only
+// register listeners and patch audio.play/pause (and `audio` is itself an imported
+// module now). Verified by walking the AST at IIFE-body depth. If a hook were ever
+// read there it would THROW loudly — see ./host.js — rather than silently misbehave.
+//
+// See ./host.js: reading an unwired hook THROWS, and tests/js/host_contract.test.js
+// fails CI if the hooks used here and the hooks app.js wires ever drift apart.
+import { audio } from './audio-el.js';
+import { _audioSeek, _songEventPayload, jucePlayer, setPlayButtonState } from './transport.js';
+import { setSpeed } from './player-controls.js';
+import { S } from './player-state.js';
+
+(function _installJuceEngineRoutingWatcher() {
+    const juceApi = window.feedBackDesktop?.audio;
+    if (!juceApi || typeof juceApi.isAudioRunning !== 'function') {
+        // Desktop bridge present but audio API incomplete — the whole
+        // exclusive reroute chain is dead and this line is the only witness.
+        // (Docker sphere has no bridge at all: stay silent, nothing to
+        // diagnose there and no debug flag to gate on.)
+        if (window.feedBackDesktop) {
+            console.log('[asio-diag] routing watcher NOT installed (audio api incomplete)');
+        }
+        return;
+    }
+
+    let _rerouteInFlight = false;
+    // URL that JUCE's loadBackingTrack *explicitly rejected* (ok === false —
+    // e.g. a codec it can't read). The poll below would otherwise retry the
+    // same doomed track every 350 ms; remember it and skip until the song
+    // changes. Only a hard JUCE reject is memoised here — transient failures
+    // (a network blip on /api/audio-local-path, an isAudioRunning() race
+    // during a device restart) are deliberately NOT memoised so they retry.
+    let _rerouteRejectedUrl = null;
+    // Exclusive-style output backends silence every other client on the
+    // endpoint — including our own <audio> element. The share mode IS the
+    // JUCE output device type: "Windows Audio (Exclusive Mode)" is a
+    // hardcoded, unlocalised JUCE type name; ASIO drivers typically hold
+    // the endpoint exclusively too. "Windows Audio (Low Latency Mode)" is
+    // shared and must NOT match.
+    function _isExclusiveOutputType(t) {
+        return t === 'Windows Audio (Exclusive Mode)' || t === 'ASIO';
+    }
+    // [feedpak-route] diagnostics: log the raw outputType string once per
+    // value change (this runs on a 350ms poll — logging every tick would
+    // flood the diagnostics buffer).
+    let _loggedOutputType;
+    // [asio-diag] verbose diagnostics, gated on --debug (preload exposes
+    // audio.debugEnabled). Resolved once at install; until it resolves the
+    // flag stays false and verbose lines are skipped. Shared with the
+    // renderer-bus feeder below via window._asioDiagEnabled.
+    let _asioDiag = false;
+    if (typeof juceApi.debugEnabled === 'function') {
+        juceApi.debugEnabled().then((v) => {
+            _asioDiag = !!v;
+            // Deferred install line: the flag resolves async, so logging at
+            // IIFE entry would race it. Change-detection isn't needed — this
+            // runs once per page load.
+            if (_asioDiag) console.log('[asio-diag] routing watcher installed');
+        }).catch(() => {});
+    }
+    window._asioDiagEnabled = () => _asioDiag;
+    async function _outputIsExclusive() {
+        if (typeof juceApi.getCurrentDevice !== 'function') {
+            if (_loggedOutputType !== '<no-getCurrentDevice>') {
+                _loggedOutputType = '<no-getCurrentDevice>';
+                console.warn('[feedpak-route] juceApi.getCurrentDevice missing — cannot detect exclusive output');
+            }
+            return false;
+        }
+        try {
+            const dev = await juceApi.getCurrentDevice();
+            const t = dev?.outputType || dev?.type || '';
+            const excl = _isExclusiveOutputType(t);
+            if (t !== _loggedOutputType) {
+                _loggedOutputType = t;
+                console.log('[feedpak-route] outputType=', JSON.stringify(t), '→ exclusive=', excl);
+                // [asio-diag] full device object on every type change — shows
+                // the exact strings the predicate saw (inputType vs outputType,
+                // device names, duplex), so a driver reporting a non-'ASIO'
+                // type name is visible in tester logs.
+                if (_asioDiag) {
+                    try {
+                        console.log('[asio-diag] getCurrentDevice=', JSON.stringify(dev));
+                    } catch (_) { /* circular/hostile object — skip */ }
+                }
+            }
+            return excl;
+        } catch (e) {
+            if (_loggedOutputType !== '<getCurrentDevice-failed>') {
+                _loggedOutputType = '<getCurrentDevice-failed>';
+                console.warn('[feedpak-route] getCurrentDevice failed:', e);
+            }
+            return false;
+        }
+    }
+    // highway.js's initial song-load routing consults this for the same
+    // feedpak-under-exclusive decision the watcher makes below.
+    window._juceOutputIsExclusive = _outputIsExclusive;
+    // Returns true when window._currentSongAudio no longer references the exact
+    // snapshot object captured at reroute entry — i.e. the song was swapped (or
+    // cleared) mid-flight. Staleness is detected by object-reference identity,
+    // not by URL value.
+    function _isStale(songAudio) {
+        return window._currentSongAudio !== songAudio;
+    }
+
+    // Migrates the loaded song from the HTML5 element onto the JUCE backing
+    // transport. Throws only on transient/unexpected failures.
+    // `songAudio` is the snapshot captured at reroute entry; if it stops being
+    // the current song mid-flight we abort without mutating global routing.
+    // Returns a distinct string outcome — the caller must NOT conflate them:
+    //   'switched' — song now plays via JUCE.
+    //   'rejected' — JUCE hard-rejected the track (codec). Caller memoises it.
+    //   'stale'    — the loaded song changed mid-flight; aborted, NOT memoised.
+    // (a transient transport-start failure throws instead — also not memoised.)
+    async function _switchHtml5ToJuce(songAudio) {
+        const url = songAudio.url;
+        const wasPlaying = S.isPlaying;
+        const pos = audio.currentTime || 0;
+        window.feedBack?.playback?.recordRouteChange?.({
+            routeKind: 'desktop-native',
+            state: 'switching',
+            preservedTime: true,
+            safeReason: 'desktop audio engine became active',
+            requesterId: 'core.juce-route',
+        });
+        // Mark a reroute in progress so the <audio> 'play'/'pause' listeners
+        // suppress their song:play / song:pause emissions: the migration is
+        // transparent — playback genuinely continues — so plugin state and
+        // window.feedBack.isPlaying must NOT flip. This also silences the
+        // "Audio paused unexpectedly" diagnostic. A REFCOUNT (not a boolean)
+        // lets an overlapping reroute's deferred release coexist: each switch
+        // increments on entry and decrements after its own timeout; listeners
+        // treat any count > 0 as "reroute active".
+        window._juceRerouteInProgress = (window._juceRerouteInProgress || 0) + 1;
+        audio.pause();
+        try {
+            const res = await fetch(`/api/audio-local-path?url=${encodeURIComponent(url)}`);
+            if (!res.ok) {
+                console.warn('[feedpak-route] audio-local-path HTTP', res.status, 'for', url);
+                throw new Error('HTTP ' + res.status);
+            }
+            const { path } = await res.json();
+            console.log('[feedpak-route] audio-local-path resolved:', (typeof path === 'string' && path.split(/[\\/]/).pop()) || '<missing>');
+            if (_isStale(songAudio)) return 'stale';   // song changed mid-fetch
+            const ok = await juceApi.loadBackingTrack(path);
+            if (ok === false) {
+                // JUCE rejected the track — stay on HTML5, resume if needed.
+                console.warn('[juce-reroute] loadBackingTrack rejected; staying on HTML5');
+                // Only resume if the element still has a source. In the normal
+                // flow audio.src is intact here, but a prior HTML5→JUCE switch
+                // clears it — re-point + load before resuming so a bounced
+                // reroute doesn't try to play() an empty element.
+                if (S.isPlaying && !_isStale(songAudio)) {
+                    if (!audio.src) { audio.src = url; audio.load(); }
+                    try { await audio.play(); } catch (_) { /* ignore */ }
+                }
+                window.feedBack?.playback?.recordRouteChange?.({
+                    routeKind: 'browser-media',
+                    state: 'degraded',
+                    preservedTime: true,
+                    safeReason: 'desktop audio route rejected track; kept browser media route',
+                    requesterId: 'core.juce-route',
+                });
+                return 'rejected';
+            }
+            if (_isStale(songAudio)) return 'stale';
+            const dur = await juceApi.getBackingDuration();
+            await juceApi.seekBacking(pos);
+            // Start the new transport BEFORE committing global routing state, so
+            // a play() failure can't leave us in "JUCE mode, nothing playing"
+            // (the silent-song state this watcher exists to prevent).
+            // jucePlayer.play() RETURNS false (it does not throw) when
+            // startBacking fails — check the result, don't just await it.
+            // A play() failure is a TRANSIENT transport-start issue, not a hard
+            // codec reject: throw (rather than returning 'rejected') so the
+            // caller's catch path handles it WITHOUT memoising the URL, leaving
+            // it free to retry on the next poll. Only 'rejected' is memoised.
+            // Re-read isPlaying as late as possible: the user can press Pause
+            // during the multi-await fetch/IPC chain above. Starting the JUCE
+            // transport off a stale `wasPlaying` snapshot would resume a song
+            // the user just paused. Only start it if playback is still wanted.
+            if (S.isPlaying) {
+                const started = await jucePlayer.play();
+                if (started === false) {
+                    if (!_isStale(songAudio) && S.isPlaying) {
+                        try { await audio.play(); } catch (_) { /* ignore */ }
+                    }
+                    throw new Error('jucePlayer.play() failed (transient transport start)');
+                }
+            }
+            if (_isStale(songAudio)) {
+                // Song changed while JUCE was spinning up — undo and bail.
+                await jucePlayer.pause().catch(() => {});
+                return 'stale';
+            }
+            if (window.jucePlayer) {
+                jucePlayer._dur = dur;
+                jucePlayer._pos = pos;
+                jucePlayer._pollAt = performance.now();
+            }
+            window._juceMode = true;
+            window._juceAudioUrl = url;
+            const _spSlider = document.getElementById?.('speed-slider');
+            if (_spSlider) setSpeed(_spSlider.value / 100);
+            audio.src = '';
+            try {
+                const apply = window.feedBack?.audio?.applySongVolume;
+                if (typeof apply === 'function') await apply();
+            } catch (_) { /* best-effort */ }
+            console.log('[juce-reroute] HTML5 → JUCE @', pos.toFixed(2), 's playing=', wasPlaying);
+            window.feedBack?.playback?.recordRouteChange?.({
+                routeKind: 'desktop-native',
+                state: 'active',
+                preservedTime: true,
+                safeReason: 'desktop audio route active',
+                requesterId: 'core.juce-route',
+            });
+            return 'switched';
+        } catch (err) {
+            // Path lookup, JSON parse, or a JUCE IPC call threw partway through.
+            // audio.pause() already ran above; restore HTML5 playback so a
+            // previously playing song isn't left silently paused, then re-throw
+            // so the caller logs it. The caller does NOT memoise this URL —
+            // transient failures must retry on the next poll.
+            if (S.isPlaying && !window._juceMode && !_isStale(songAudio)) {
+                if (!audio.src) { audio.src = url; audio.load(); }
+                try { await audio.play(); } catch (_) { /* ignore */ }
+            }
+            window.feedBack?.playback?.recordRouteChange?.({
+                routeKind: 'browser-media',
+                state: 'degraded',
+                preservedTime: true,
+                safeReason: 'desktop audio route failed; kept browser media route',
+                requesterId: 'core.juce-route',
+            });
+            throw err;
+        } finally {
+            // Clearing audio.src above dispatches a 'pause' event in a later
+            // task, after this synchronous finally. Defer the refcount
+            // decrement so that trailing event is still suppressed; a 0ms
+            // timeout lands after the pending pause-event task. Decrementing
+            // (rather than zeroing) leaves any overlapping reroute's own
+            // suppression intact.
+            setTimeout(() => {
+                window._juceRerouteInProgress = Math.max(
+                    0, (window._juceRerouteInProgress || 1) - 1);
+            }, 0);
+        }
+    }
+
+    async function _switchJuceToHtml5(songAudio) {
+        const url = songAudio.url;
+        const wasPlaying = S.isPlaying;
+        const pos = (window.jucePlayer ? jucePlayer.currentTime : 0) || 0;
+        window.feedBack?.playback?.recordRouteChange?.({
+            routeKind: 'browser-media',
+            state: 'switching',
+            preservedTime: true,
+            safeReason: 'desktop audio engine stopped',
+            requesterId: 'core.juce-route',
+        });
+        // Mark a reroute in progress (refcount) so the <audio> 'play' listener
+        // suppresses its song:play emission — the migration is transparent and
+        // playback genuinely continues, so plugin state must not flip. Held
+        // until after the (possibly deferred) audio.play() event has fired.
+        window._juceRerouteInProgress = (window._juceRerouteInProgress || 0) + 1;
+        let _suppressionReleased = false;
+        const _releaseSuppression = () => {
+            if (_suppressionReleased) return;
+            _suppressionReleased = true;
+            // Defer so the 'play' (or 'pause') event task fires while still
+            // suppressed; a 0ms timeout lands after it.
+            setTimeout(() => {
+                window._juceRerouteInProgress = Math.max(
+                    0, (window._juceRerouteInProgress || 1) - 1);
+            }, 0);
+        };
+        let _resumeScheduled = false;
+        try {
+            await jucePlayer.pause().catch(() => {});
+            if (_isStale(songAudio)) return;           // song changed mid-pause
+            window._juceMode = false;
+            window._juceAudioUrl = null;
+            audio.src = url;
+            audio.load();
+            const _spSlider = document.getElementById?.('speed-slider');
+            if (_spSlider) setSpeed(_spSlider.value / 100);
+            // Resume only AFTER the seek so playback starts at `pos`, not at 0
+            // with an audible jump once metadata arrives.
+            const resumeAtPos = () => {
+                try {
+                    // The metadata event can land after a fast song switch —
+                    // bail before touching currentTime so a stale callback
+                    // doesn't seek the newly loaded song to the old position.
+                    if (_isStale(songAudio)) return;
+                    try { audio.currentTime = pos; } catch (_) { /* ignore */ }
+                    // Re-read isPlaying (not the entry snapshot): the user may
+                    // have pressed Pause during jucePlayer.pause()/metadata
+                    // load — don't resume a song they just paused.
+                    if (S.isPlaying) {
+                        audio.play().catch(() => { /* ignore */ });
+                    }
+                } finally {
+                    _releaseSuppression();
+                }
+            };
+            _resumeScheduled = true;
+            if (audio.readyState >= 1) {
+                resumeAtPos();
+            } else {
+                // Wait for metadata to resume at `pos`. But metadata may never
+                // arrive (bad URL, network error) — that would leak the
+                // suppression refcount and permanently silence song:play /
+                // song:pause. Guard with the element's 'error' event AND a
+                // backstop timeout; whichever fires first wins, the others are
+                // detached. _releaseSuppression is idempotent regardless.
+                let _settled = false;
+                const _onMeta = () => { finish(true); };
+                const _onErr = () => { finish(false); };
+                let _backstop;
+                function finish(reachedMetadata) {
+                    if (_settled) return;
+                    _settled = true;
+                    clearTimeout(_backstop);
+                    audio.removeEventListener('loadedmetadata', _onMeta);
+                    audio.removeEventListener('error', _onErr);
+                    if (reachedMetadata) {
+                        resumeAtPos();             // resumeAtPos releases suppression
+                    } else {
+                        _releaseSuppression();     // no resume — just release
+                    }
+                }
+                audio.addEventListener('loadedmetadata', _onMeta, { once: true });
+                audio.addEventListener('error', _onErr, { once: true });
+                // 10s is well beyond a normal local-file metadata load.
+                _backstop = setTimeout(() => { finish(false); }, 10000);
+            }
+        } finally {
+            // resumeAtPos owns the release once scheduled; if we returned
+            // early (stale, before scheduling) release here instead.
+            // _releaseSuppression is idempotent so an overlap is harmless.
+            if (!_resumeScheduled) _releaseSuppression();
+        }
+        try {
+            const apply = window.feedBack?.audio?.applySongVolume;
+            if (typeof apply === 'function') await apply();
+        } catch (_) { /* best-effort */ }
+        console.log('[juce-reroute] JUCE → HTML5 @', pos.toFixed(2), 's playing=', wasPlaying);
+        window.feedBack?.playback?.recordRouteChange?.({
+            routeKind: 'browser-media',
+            state: 'active',
+            preservedTime: true,
+            safeReason: 'browser media route active',
+            requesterId: 'core.juce-route',
+        });
+    }
+
+    async function _reevaluateJuceRouting() {
+        if (_rerouteInFlight) return;
+        const songAudio = window._currentSongAudio;
+        // /audio/ songs are always JUCE-routable. A feedpak full-mix
+        // (single-mix pack, no stems) is routable ONLY under an
+        // exclusive-style output — in shared mode it must stay on HTML5 so
+        // the stem mixer / WebAudio path keeps working. Sloppak stem URLs
+        // are never routable (per-stem mix can't ride a single transport).
+        if (!songAudio || (!songAudio.juceEligible && !songAudio.feedpakFullMix)) return;
+        // Don't race highway.js's own initial song-load routing: it owns
+        // _juceMode until _juceRoutingPromise settles. Re-running our switch
+        // concurrently would double-call loadBackingTrack for the same URL.
+        if (window._highwayJuceRoutingPending) return;
+
+        // Claim the in-flight guard SYNCHRONOUSLY, before the first await. The
+        // watcher is driven by a 350ms setInterval; if isAudioRunning() (or any
+        // later await) stalls past the poll period, a second tick would
+        // otherwise pass the `if (_rerouteInFlight) return` check above and run
+        // a concurrent switch — duplicate loadBackingTrack IPCs racing on
+        // _juceMode / audio.src. Setting it here closes that window.
+        _rerouteInFlight = true;
+        try {
+            let running;
+            try { running = await juceApi.isAudioRunning(); }
+            catch (_) { return; }
+            if (_isStale(songAudio)) return;               // song changed during IPC
+            // Eligibility is evaluated per tick, not snapshotted at song load:
+            // the output share mode can change mid-song (device switch in the
+            // Audio Engine panel), and a feedpak full-mix must follow it —
+            // exclusive → ride the engine; back to shared → return to HTML5.
+            let eligible = !!songAudio.juceEligible;
+            if (!eligible && songAudio.feedpakFullMix && running) {
+                eligible = await _outputIsExclusive();
+                if (_isStale(songAudio)) return;           // song changed during IPC
+            }
+            const wantJuce = !!(running && eligible);
+            // [feedpak-route] diagnostics: one line per decision change (the
+            // watcher polls at 350ms; steady state must not spam the buffer).
+            const _decision = 'running=' + running + ' eligible=' + eligible
+                + ' feedpakFullMix=' + !!songAudio.feedpakFullMix
+                + ' juceMode=' + !!window._juceMode + ' url=' + songAudio.url;
+            if (_decision !== window._lastFeedpakRouteDecision) {
+                window._lastFeedpakRouteDecision = _decision;
+                console.log('[feedpak-route] watcher:', _decision);
+            }
+            if (wantJuce === !!window._juceMode) return;   // routing already consistent
+            // Don't keep retrying a track JUCE explicitly rejected.
+            if (wantJuce && songAudio.url === _rerouteRejectedUrl) return;
+
+            if (wantJuce) {
+                const outcome = await _switchHtml5ToJuce(songAudio);
+                // Memoise ONLY an explicit hard JUCE reject. A successful
+                // switch clears the memo; a 'stale' abort (song changed
+                // mid-flight) leaves it untouched — it must never be
+                // misclassified as a reject, even if the song object was
+                // swapped and then restored before this point.
+                if (outcome === 'rejected') {
+                    _rerouteRejectedUrl = songAudio.url;
+                } else if (outcome === 'switched') {
+                    _rerouteRejectedUrl = null;
+                }
+                // outcome === 'stale': leave _rerouteRejectedUrl as-is.
+            } else {
+                await _switchJuceToHtml5(songAudio);
+                // The engine stopped (or a feedpak's output left exclusive
+                // mode). Clear any hard-reject memo so a later engine restart
+                // or mode change re-evaluates the track at least once — the
+                // rejection may have been a transient device/decoder state.
+                _rerouteRejectedUrl = null;
+            }
+        } catch (e) {
+            // Transient failure — log but do NOT memoise, so the next poll retries.
+            console.warn('[juce-reroute] re-route failed (will retry):', e);
+        } finally {
+            _rerouteInFlight = false;
+        }
+    }
+    window._reevaluateJuceRouting = _reevaluateJuceRouting;
+
+    // Clears the hard-reject memo. Called from the song-teardown sites that
+    // null window._currentSongAudio (showScreen, playSong) so that reloading
+    // the same file later gets a fresh routing attempt — a prior reject may
+    // have been a transient JUCE/device state, not a permanent codec issue.
+    window._clearJuceRerouteMemo = function () { _rerouteRejectedUrl = null; };
+
+    // The engine can be started/stopped from several places (the desktop Audio
+    // Engine panel, the audio_engine plugin, note_detect) and via setDevice
+    // restarts — and the contextBridge api object is frozen, so its methods
+    // can't be wrapped. Poll isAudioRunning() while a song is loaded; the check
+    // is a cheap IPC boolean and no-ops once routing is already consistent.
+    // Skip the poll while the document is hidden (background tab / minimised
+    // window) — engine toggles there will be reconciled on the first poll
+    // after the tab is visible again.
+    setInterval(() => {
+        if (document.hidden) return;
+        if (window._currentSongAudio) void _reevaluateJuceRouting();
+    }, 350);
+})();
+
+// Renderer-audio bus feeder (desktop Phase 2): when the engine holds the
+// output endpoint in an exclusive-style mode, Chromium cannot reach the
+// device, so any song audio still played by the renderer goes silent. The
+// Phase 1 watcher above already migrates what a single-file transport can
+// carry (loose /audio/ songs, feedpak full-mixes) onto the native backing
+// transport. This feeder covers the rest — the stems plugin's multi-stem
+// WebAudio graph, plus <audio>-element songs the native transport could not
+// take (e.g. a codec loadBackingTrack rejected).
+//
+// Mechanism: capture the renderer-side master with an AudioWorklet tap,
+// re-point the owning AudioContext at a null sink so it keeps rendering
+// without a device, and push ~10 ms chunks over IPC into the engine's
+// renderer bus, where they are mixed into the exclusive output like a
+// backing track (~10-20 ms added latency on song audio only; the guitar
+// monitoring path is untouched). Validated by the fix12 tester spike:
+// null-sink rendering works, clocks hold (drift → 0), no overflow.
+//
+// Docker sphere: window.feedBackDesktop is undefined → this whole block is
+// inert. Shared-mode desktop: the bus stays disabled (no double audio) and
+// captured contexts keep/regain their default sink.
+(function _installRendererBusFeeder() {
+    const api = window.feedBackDesktop?.audio;
+    if (!api || typeof api.setRendererBus !== 'function'
+             || typeof api.pushRendererAudio !== 'function') {
+        // Silent in the Docker sphere (no bridge, no debug flag); a desktop
+        // bridge missing the bus API is the diagnostic case.
+        if (window.feedBackDesktop) {
+            console.log('[asio-diag] renderer-bus feeder NOT installed (api=' + !!api
+                + ' setRendererBus=' + typeof api?.setRendererBus
+                + ' pushRendererAudio=' + typeof api?.pushRendererAudio + ')');
+        }
+        return;
+    }
+    // Deferred like the watcher's install line: gate on the async debug flag.
+    if (typeof api.debugEnabled === 'function') {
+        api.debugEnabled().then((v) => {
+            if (v) console.log('[asio-diag] renderer-bus feeder installed (loopback-capable='
+                + (typeof window.navigator?.mediaDevices?.getDisplayMedia === 'function') + ')');
+        }).catch(() => {});
+    }
+
+    const TAP_WORKLET = `
+        class FeedbackBusTap extends AudioWorkletProcessor {
+            process(inputs) {
+                const inp = inputs[0];
+                if (inp && inp[0]) {
+                    const L = inp[0], R = inp[1] || inp[0];
+                    const out = new Float32Array(L.length * 2);
+                    for (let i = 0; i < L.length; i++) { out[i*2] = L[i]; out[i*2+1] = R[i]; }
+                    this.port.postMessage(out, [out.buffer]);
+                }
+                return true;
+            }
+        }
+        registerProcessor('feedback-bus-tap', FeedbackBusTap);
+    `;
+    const _tapModuleUrl = URL.createObjectURL(new Blob([TAP_WORKLET], { type: 'application/javascript' }));
+    const _tapModuleLoaded = new WeakSet();   // AudioContexts with the module added
+
+    // One tap per captured graph. `active` gates the push (the worklet keeps
+    // running when inactive — it's silent bookkeeping, not audio).
+    function _makeTap(ctx) {
+        const state = { node: null, active: false, batch: [], batchFrames: 0 };
+        state.attach = async (sourceNode) => {
+            if (!_tapModuleLoaded.has(ctx)) {
+                await ctx.audioWorklet.addModule(_tapModuleUrl);
+                _tapModuleLoaded.add(ctx);
+            }
+            if (!state.node) {
+                state.node = new AudioWorkletNode(ctx, 'feedback-bus-tap', { numberOfInputs: 1, channelCount: 2 });
+                const BATCH = Math.round(ctx.sampleRate / 100);   // ~10 ms
+                state.node.port.onmessage = (e) => {
+                    if (!state.active) { state.batch = []; state.batchFrames = 0; return; }
+                    state.batch.push(e.data);
+                    state.batchFrames += e.data.length / 2;
+                    if (state.batchFrames >= BATCH) {
+                        const merged = new Float32Array(state.batchFrames * 2);
+                        let o = 0;
+                        for (const c of state.batch) { merged.set(c, o); o += c.length; }
+                        api.pushRendererAudio(merged, ctx.sampleRate);
+                        state.batch = []; state.batchFrames = 0;
+                    }
+                };
+            }
+            sourceNode.connect(state.node);
+            // No onward connection: the tap is a sink-side observer; audibility
+            // in shared mode comes from the graph's own destination path.
+        };
+        state.detach = (sourceNode) => {
+            state.active = false;
+            state.batch = []; state.batchFrames = 0;
+            if (state.node && sourceNode) {
+                try { sourceNode.disconnect(state.node); } catch (_) { /* already gone */ }
+            }
+        };
+        return state;
+    }
+
+    // ── Core <audio> element capture ─────────────────────────────────────────
+    // createMediaElementSource permanently reroutes the element into its
+    // context, so it is created lazily — only the first time an exclusive
+    // device actually needs it — and never torn down. From then on the element
+    // always plays through _elCtx; sink toggling routes it to the speakers
+    // (shared mode) or the null sink + bus (exclusive mode).
+    let _elCtx = null, _elSource = null, _elTap = null;
+    async function _ensureElementCapture() {
+        if (_elCtx) return;
+        const el = document.getElementById('audio');
+        if (!el) throw new Error('no core audio element');
+        // Assign the module state ONLY after the whole chain succeeded.
+        // createMediaElementSource throws InvalidStateError when another
+        // consumer (highway_3d's analyser tap) already owns the element's
+        // one-shot source — assigning _elCtx before that throw poisoned every
+        // later tick into `_elTap.active` TypeErrors (tester log 2026-07-11)
+        // while the song kept playing on the default device.
+        const ctx = new AudioContext();
+        let source, tap;
+        try {
+            source = ctx.createMediaElementSource(el);
+            source.connect(ctx.destination);
+            tap = _makeTap(ctx);
+            await tap.attach(source);
+        } catch (e) {
+            try { await ctx.close(); } catch (_) { /* already closed */ }
+            throw e;
+        }
+        _elCtx = ctx; _elSource = source; _elTap = tap;
+    }
+
+    // ── Whole-app loopback capture ───────────────────────────────────────────
+    // Preferred mode: one getDisplayMedia frame-audio capture covers EVERY
+    // sound the app makes (song, previews, UI) — no per-surface taps, so
+    // plugin-private AudioContexts (song-preview, future plugins) survive
+    // exclusive/ASIO output too. The desktop main process answers the request
+    // with this window's own frame (frame-scoped — no other apps' audio).
+    // Local playback is silenced via the suppressLocalAudioPlayback track
+    // constraint, with a page-mute IPC fallback (capture taps frame audio
+    // before the output mute, so a muted page still feeds the stream).
+    let _lbStream = null, _lbCtx = null, _lbTap = null, _lbPageMuted = false;
+    let _loopbackUnavailable = false;   // sticky: probe once, then fall back
+    async function _engageLoopback() {
+        const stream = await navigator.mediaDevices.getDisplayMedia({
+            video: true,
+            audio: { suppressLocalAudioPlayback: true },
+        });
+        for (const t of stream.getVideoTracks()) t.stop();   // required, unused
+        const track = stream.getAudioTracks()[0];
+        if (!track) {
+            for (const t of stream.getTracks()) t.stop();
+            throw new Error('no loopback audio track');
+        }
+        try {
+            // Fresh context per session (not reused) so teardown's close()
+            // fully releases the tap worklet node — see _teardownLoopback.
+            _lbCtx = new AudioContext();
+            if (_lbCtx.state !== 'running') await _lbCtx.resume().catch(() => {});
+            const source = _lbCtx.createMediaStreamSource(stream);
+            const tap = _makeTap(_lbCtx);
+            await tap.attach(source);
+            const suppressed = track.getSettings?.().suppressLocalAudioPlayback === true;
+            if (!suppressed && typeof api.setPageMuted === 'function') {
+                _lbPageMuted = (await api.setPageMuted(true)) === true;
+            }
+            if (window._asioDiagEnabled?.()) {
+                console.log('[asio-diag] loopback: suppressed=', suppressed,
+                    'pageMuted=', _lbPageMuted, 'rate=', _lbCtx.sampleRate);
+            }
+            await api.setRendererBus(true, 1.0);
+            tap.active = true;
+            _lbStream = stream; _lbTap = tap;
+            _mode = 'loopback';
+            console.log('[renderer-bus] engaged: app loopback → engine bus');
+        } catch (e) {
+            for (const t of stream.getTracks()) t.stop();
+            throw e;
+        }
+    }
+    async function _teardownLoopback() {
+        if (_lbTap) _lbTap.active = false;
+        if (_lbStream) for (const t of _lbStream.getTracks()) t.stop();
+        _lbStream = null; _lbTap = null;
+        // Close the capture context so its tap worklet node is released. The
+        // context is per-session (not reused): without this, each exclusive⇄
+        // shared switch orphaned a live worklet on a long-lived context.
+        if (_lbCtx) {
+            try { await _lbCtx.close(); } catch (_) { /* already closed */ }
+            _lbCtx = null;
+        }
+        if (_lbPageMuted && typeof api.setPageMuted === 'function') {
+            try { await api.setPageMuted(false); } catch (_) { /* engine gone */ }
+        }
+        _lbPageMuted = false;
+    }
+
+    // ── Engagement state machine ─────────────────────────────────────────────
+    // 'off' | 'loopback' | 'element' | 'stems' (element/stems = fallback when
+    // loopback capture is unavailable: old desktop main, denied capture)
+    let _mode = 'off';
+    let _stemsGraph = null;   // { context, masterNode } snapshot while engaged
+    let _stemsTap = null;
+    const _stemsTaps = new WeakMap();  // context → tap (stems ctx is reused across songs)
+    let _busy = false;
+
+    async function _setSink(ctx, exclusive) {
+        if (typeof ctx.setSinkId !== 'function') throw new Error('setSinkId unsupported');
+        await ctx.setSinkId(exclusive ? { type: 'none' } : '');
+        if (ctx.state !== 'running') await ctx.resume().catch(() => {});
+        // [asio-diag] a context left on the default sink while the bus is
+        // engaged is exactly the "song on the wrong device" symptom — record
+        // every successful sink flip (failures throw and are logged upstream).
+        if (window._asioDiagEnabled?.()) {
+            console.log('[asio-diag] setSink:', exclusive ? 'null-sink' : 'default',
+                'state=', ctx.state, 'rate=', ctx.sampleRate);
+        }
+    }
+
+    async function _disengage() {
+        if (_mode === 'off') return;
+        const prev = _mode;
+        _mode = 'off';
+        try { await api.setRendererBus(false, 0); } catch (_) { /* engine gone */ }
+        if (prev === 'loopback') {
+            await _teardownLoopback();
+        } else if (prev === 'element' && _elCtx) {
+            _elTap.active = false;
+            await _setSink(_elCtx, false).catch(() => {});
+        } else if (prev === 'stems' && _stemsGraph) {
+            if (_stemsTap) _stemsTap.detach(_stemsGraph.masterNode);
+            await _setSink(_stemsGraph.context, false).catch(() => {});
+            _stemsGraph = null; _stemsTap = null;
+        }
+        console.log('[renderer-bus] disengaged (' + prev + ')');
+    }
+
+    async function _engageStems(graph) {
+        await _setSink(graph.context, true);
+        let tap = _stemsTaps.get(graph.context);
+        if (!tap) { tap = _makeTap(graph.context); _stemsTaps.set(graph.context, tap); }
+        await tap.attach(graph.masterNode);
+        await api.setRendererBus(true, 1.0);
+        tap.active = true;
+        _stemsGraph = graph; _stemsTap = tap;
+        _mode = 'stems';
+        console.log('[renderer-bus] engaged: stems graph → engine bus');
+    }
+
+    async function _engageElement() {
+        await _ensureElementCapture();
+        await _setSink(_elCtx, true);
+        await api.setRendererBus(true, 1.0);
+        _elTap.active = true;
+        _mode = 'element';
+        console.log('[renderer-bus] engaged: <audio> element → engine bus');
+    }
+
+    async function _reevaluate() {
+        if (_busy) return;
+        _busy = true;
+        try {
+            let running = false, exclusive = false;
+            try {
+                running = await api.isAudioRunning();
+            } catch (_) { /* engine unreachable → treat as not running */ }
+            if (running) {
+                // Reuse the Phase 1 predicate installed by the routing watcher
+                // (getCurrentDevice + exclusive-type check with change-logged
+                // diagnostics). Fail closed if it is somehow absent.
+                exclusive = !!(await window._juceOutputIsExclusive?.());
+            }
+
+            // The stems plugin publishes its live graph while a multi-stem
+            // song is loaded (and removes it on teardown).
+            const stems = (window.feedBack || window.slopsmith)?.stems?.audioGraph || null;
+            // Element songs: a song is loaded, it is NOT riding the native
+            // transport (Phase 1 owns those), and the stems graph is not the
+            // player. Covers native-transport rejects (codec) in exclusive
+            // mode — without this they would be silent.
+            const songAudio = window._currentSongAudio;
+            const elementSong = !!songAudio && !window._juceMode && !stems;
+
+            let want = 'off';
+            if (running && exclusive) {
+                // Loopback covers ALL app audio (song, previews, UI), so it
+                // engages for the whole exclusive session — not just while a
+                // song is loaded. Per-surface modes remain as fallback when
+                // loopback capture is unavailable (old desktop main without
+                // the display-media handler, capture denied).
+                if (!_loopbackUnavailable) want = 'loopback';
+                else if (stems) want = 'stems';
+                else if (elementSong) want = 'element';
+            }
+            // Song audio riding the native transport must not ALSO ride the
+            // loopback (double-carry into the same engine output). The native
+            // transport plays from the engine, not the page, so page loopback
+            // never hears it — no conflict; loopback stays engaged for
+            // previews/UI while the transport owns the song.
+
+            // [asio-diag] full decision vector, change-gated (500ms poll —
+            // steady state must not flood the buffer). This is the feeder-side
+            // counterpart of the watcher's [feedpak-route] decision line: it
+            // shows WHY the bus did or didn't engage (exclusive predicate,
+            // stems graph presence, native transport ownership, element song).
+            if (window._asioDiagEnabled?.()) {
+                const d = 'running=' + running + ' exclusive=' + exclusive
+                    + ' stems=' + !!stems + ' songAudio=' + !!songAudio
+                    + ' juceMode=' + !!window._juceMode
+                    + ' elementSong=' + elementSong
+                    + ' loopbackUnavailable=' + _loopbackUnavailable
+                    + ' want=' + want + ' mode=' + _mode;
+                if (d !== window._lastRendererBusDecision) {
+                    window._lastRendererBusDecision = d;
+                    console.log('[asio-diag] renderer-bus:', d);
+                }
+            }
+
+
+            const stemsGraphChanged = _mode === 'stems' && stems !== _stemsGraph;
+            if (want !== _mode || stemsGraphChanged) {
+                await _disengage();
+                try {
+                    if (want === 'loopback') await _engageLoopback();
+                    else if (want === 'stems') await _engageStems(stems);
+                    else if (want === 'element') await _engageElement();
+                } catch (e) {
+                    if (want === 'loopback') {
+                        // Capture unavailable (no handler in an old desktop
+                        // main, permission denied) — remember and fall back to
+                        // the per-surface modes on the next tick.
+                        _loopbackUnavailable = true;
+                        console.warn('[renderer-bus] loopback capture unavailable — falling back to surface taps:', e);
+                    }
+                    throw e;
+                }
+            }
+        } catch (e) {
+            // Explicit name/message/stack head — the console-message forward
+            // stringifies a DOMException to the useless "[object DOMException]".
+            console.warn('[renderer-bus] reevaluate failed (will retry):',
+                (e && e.name ? e.name + ': ' + e.message : String(e)),
+                (e && e.stack ? '| ' + String(e.stack).split('\n')[1] : ''));
+            _mode = 'off';
+            // A partial engage may have left the bus enabled with no producer
+            // and the page muted — undo both so a failed tick can't strand
+            // audio in silence until the next successful engage.
+            try { await api.setRendererBus(false, 0); } catch (_) { /* engine gone */ }
+            await _teardownLoopback().catch(() => {});
+        } finally {
+            _busy = false;
+        }
+    }
+
+    // Same cadence/rationale as the routing watcher above. Also re-check on
+    // visibility return so a device switch made while hidden is reconciled.
+    setInterval(() => { if (!document.hidden) void _reevaluate(); }, 500);
+    document.addEventListener('visibilitychange', () => { if (!document.hidden) void _reevaluate(); });
+    window._reevaluateRendererBus = _reevaluate;
+})();
+
+// Desktop JUCE backing uses an empty <audio> element; plugins such as Section Map
+// still seek via audio.currentTime / pause / play. Mirror those onto jucePlayer
+// while _juceMode is active. Same-tick pause+seek coalesce into a single seek
+// (no stopBacking before seek — HTML5 needed that for buffering; JUCE does not).
+export let _resetJuceAudioShimChain = function () {};
+(function _installJuceAudioElementShim() {
+    if (!window.feedBackDesktop?.audio) return;
+
+    const mediaProto = HTMLMediaElement.prototype;
+    const ctDesc = Object.getOwnPropertyDescriptor(mediaProto, 'currentTime');
+    const pausedDesc = Object.getOwnPropertyDescriptor(mediaProto, 'paused');
+    if (!ctDesc?.get || !ctDesc?.set || !pausedDesc?.get) return;
+
+    const nativePlay = mediaProto.play;
+    const nativePause = mediaProto.pause;
+
+    let chain = Promise.resolve();
+    /** Same-tick pause + seek (Section Map): coalesce to one seek — no stopBacking before seek. */
+    let _juceShimBatch = null;
+    let _juceShimBatchFlushScheduled = false;
+    let _juceShimGen = 0;
+    function enqueue(fn) {
+        const gen = _juceShimGen;
+        const p = chain.then(async () => {
+            if (gen !== _juceShimGen) return;
+            return fn(gen);
+        });
+        chain = p.catch((e) => {
+            console.warn('[juce-audio-shim]', e);
+        });
+        return p;
+    }
+    // forUpcomingPlay: caller will enqueue a play() right after, so don't
+    // emit pause-state side effects for a wantsPause batch — play() will
+    // overwrite them anyway.
+    function flushJuceShimBatchNow({ forUpcomingPlay = false } = {}) {
+        _juceShimBatchFlushScheduled = false;
+        const batch = _juceShimBatch;
+        _juceShimBatch = null;
+        if (!batch || !window._juceMode) return;
+        const wantsPause = !!batch.wantsPause;
+        const seekTime = batch.seekTime;
+        if (wantsPause && seekTime !== undefined) {
+            enqueue(async (gen) => {
+                const r = await _audioSeek(seekTime, 'audio-element-shim');
+                if (!r.completed) return; // seek cancelled by teardown
+                if (gen !== _juceShimGen) return;
+                if (!forUpcomingPlay) {
+                    await jucePlayer.pause();
+                    if (gen !== _juceShimGen) return;
+                    S.isPlaying = false;
+                    setPlayButtonState(false);
+                    const sm = window.feedBack;
+                    if (sm) {
+                        sm.isPlaying = false;
+                        sm.emit('song:pause', _songEventPayload());
+                    }
+                }
+                audio.dispatchEvent(new Event('seeked'));
+            });
+            return;
+        }
+        if (wantsPause) {
+            enqueue(async (gen) => {
+                await jucePlayer.pause();
+                if (gen !== _juceShimGen) return;
+                S.isPlaying = false;
+                setPlayButtonState(false);
+                const sm = window.feedBack;
+                if (sm) {
+                    sm.isPlaying = false;
+                    sm.emit('song:pause', _songEventPayload());
+                }
+            });
+            return;
+        }
+        if (seekTime !== undefined) {
+            enqueue(async (gen) => {
+                const r = await _audioSeek(seekTime, 'audio-element-shim');
+                if (!r.completed) return; // seek cancelled by teardown
+                if (gen !== _juceShimGen) return;
+                audio.dispatchEvent(new Event('seeked'));
+            });
+        }
+    }
+    function scheduleJuceShimBatchFlush() {
+        if (_juceShimBatchFlushScheduled) return;
+        _juceShimBatchFlushScheduled = true;
+        const flushGen = _juceShimGen;
+        queueMicrotask(() => {
+            if (flushGen !== _juceShimGen) {
+                _juceShimBatchFlushScheduled = false;
+                return;
+            }
+            flushJuceShimBatchNow();
+        });
+    }
+    _resetJuceAudioShimChain = function () {
+        chain = Promise.resolve();
+        _juceShimBatch = null;
+        _juceShimBatchFlushScheduled = false;
+        _juceShimGen++;
+    };
+
+    Object.defineProperty(audio, 'currentTime', {
+        get() {
+            if (window._juceMode) return jucePlayer.currentTime;
+            return ctDesc.get.call(this);
+        },
+        set(v) {
+            if (window._juceMode) {
+                const t = Math.max(0, Number(v) || 0);
+                _juceShimBatch = _juceShimBatch || {};
+                _juceShimBatch.seekTime = t;
+                scheduleJuceShimBatchFlush();
+                return;
+            }
+            ctDesc.set.call(this, v);
+        },
+        configurable: true,
+    });
+
+    Object.defineProperty(audio, 'paused', {
+        get() {
+            if (window._juceMode) return !S.isPlaying;
+            return pausedDesc.get.call(this);
+        },
+        configurable: true,
+    });
+
+    audio.pause = function () {
+        if (window._juceMode) {
+            _juceShimBatch = _juceShimBatch || {};
+            _juceShimBatch.wantsPause = true;
+            scheduleJuceShimBatchFlush();
+            return;
+        }
+        nativePause.call(audio);
+    };
+
+    audio.play = function () {
+        if (window._juceMode) {
+            if (_juceShimBatch != null) flushJuceShimBatchNow({ forUpcomingPlay: true });
+            const p = enqueue(async (gen) => {
+                const started = await jucePlayer.play();
+                if (gen !== _juceShimGen || !started) return;
+                S.isPlaying = true;
+                setPlayButtonState(true);
+                const sm = window.feedBack;
+                if (sm) {
+                    sm.isPlaying = true;
+                    const payload = _songEventPayload();
+                    sm.emit('song:play', payload);
+                    sm.emit('song:resume', payload);
+                }
+            });
+            return p.then(() => undefined);
+        }
+        return nativePlay.call(audio);
+    };
+})();

static/js/library-state.js

@@ -0,0 +1,29 @@
+// Shared, MUTABLE library state.
+//
+// WHY A CONTAINER AND NOT PLAIN EXPORTS. An imported binding is READ-ONLY:
+// `import { _treeStats }; _treeStats = x` throws. Of the library module's 28 outward
+// bindings, 23 are only ever READ from outside, so they stay plain exports. These five
+// are genuinely WRITTEN from outside — by showScreen (session teardown bumps the epoch,
+// resets the page), deleteSongFromModal, and syncLibrarySong, none of which can move into
+// the library module because they reach the playSong/showScreen core.
+//
+// So exactly these five move onto an object, and no more. `L.treeStats = x` is a property
+// write, which works from any module holding the same `L`. Same shape as ./player-state.js.
+//
+// Add to it when a carve actually needs it, not before — a container is a shared mutable
+// global with better manners, and every field on it is a coupling you have to keep true.
+export const L = {
+    /** Library tree stats (artist -> counts), cached from /api/library/tree-stats. */
+    treeStats: null,
+    /** Same, for the favourites tree. */
+    favTreeStats: null,
+    /** Tuning names, cached from /api/library/tuning-names. */
+    tuningNames: null,
+    /**
+     * Session generation for the library. Bumped on teardown so an in-flight page fetch
+     * that resolves against a stale library can't render into the new one.
+     */
+    libEpoch: 0,
+    /** Current grid page (0-based). */
+    currentPage: 0,
+};

static/js/loops.js

@@ -0,0 +1,263 @@
+// The A–B loop — set / clear / persist, and the saved-loops list.
+//
+// The second slice out of app.js's strongly-connected core, and it owns the loop
+// state: loopA, loopB, _loopMutationGen. Nothing outside this module writes them
+// (restartCurrentSong() looked like it did, but it declares its own local shadows).
+//
+// DIRECTION MATTERS HERE. loops and section-practice are mutually dependent — the
+// SCC in miniature. clearLoop() has to drop section-practice's selection, and
+// practiceSection() has to call setLoop(). Both directions cannot be imports or the
+// no-cycle gate (rightly) rejects it. So the edge is oriented:
+//
+//     section-practice  ->  reaches loops through the HOST SEAM (host.setLoop, …)
+//     loops             ->  imports section-practice DIRECTLY
+//
+// section-practice is the higher-level feature — it is a consumer of loops, not the
+// other way round — so it is the one that gets the indirection. app.js wires this
+// module's exports into the seam for it.
+//
+// See ./host.js: reading an unwired hook THROWS, and tests/js/host_contract.test.js
+// fails CI if the hooks used here and the hooks app.js wires ever drift apart.
+import { esc, uiPrompt } from './dom.js';
+import { _audioSeek, _audioTime } from './transport.js';
+import { formatTime } from './format.js';
+import { host } from './host.js';
+import {
+    _setSectionPracticeMode,
+    _syncSectionPracticeFromLoop,
+    _updateSectionPracticeHighlight,
+    practiceSection,
+    resetSelection,
+} from './section-practice.js';
+
+// ── A-B Loop ────────────────────────────────────────────────────────────
+export let loopA = null;
+export let loopB = null;
+// Bumped on every NON-practiceSection loop mutation (direct setLoop from Saved
+// Loops / the plugin API, and clearLoop). practiceSection() captures it and bails
+// if it changes mid-retry, so a stale section retry can't overwrite a loop the
+// user just set/cleared by another path. practiceSection's own setLoop calls pass
+// skipSectionSync and do NOT bump it (they must not supersede themselves).
+export let _loopMutationGen = 0;
+
+export function setLoopStart() {
+    loopA = _audioTime();
+    document.getElementById('btn-loop-a').className = 'px-3 py-1.5 bg-green-900/50 rounded-lg text-xs text-green-300 transition';
+    updateLoopUI();
+}
+
+export function setLoopEnd() {
+    if (loopA === null) return;
+    loopB = _audioTime();
+    if (loopB <= loopA) { loopB = null; return; }
+    document.getElementById('btn-loop-b').className = 'px-3 py-1.5 bg-green-900/50 rounded-lg text-xs text-green-300 transition';
+    updateLoopUI();
+    // Manual A/B arming is a loop mutation like setLoop()'s — emit the same
+    // transport event so event-driven consumers (note_detect drill sync) see
+    // button-armed loops without having to poll getLoop().
+    window.feedBack?.playback?.transportEvent?.('loop-set', { requesterId: 'core.loop', loopA, loopB, loop: { startTime: loopA, endTime: loopB, enabled: true, state: 'active' } });
+}
+
+export function clearLoop(options) {
+    const { emitTransportEvent = true } = options || {};
+    // playSong() clears the loop on every song load, so only signal a
+    // loop-cleared transport event when a loop was actually active —
+    // otherwise every song switch emits a spurious playback:loop-cleared.
+    const hadLoop = loopA !== null || loopB !== null;
+    _setSectionPracticeMode(false, { skipClearLoop: true });
+    loopA = null;
+    loopB = null;
+    document.getElementById('btn-loop-a').className = 'px-3 py-1.5 bg-dark-600 hover:bg-dark-500 rounded-lg text-xs text-gray-300 transition';
+    document.getElementById('btn-loop-b').className = 'px-3 py-1.5 bg-dark-600 hover:bg-dark-500 rounded-lg text-xs text-gray-300 transition';
+    document.getElementById('btn-loop-clear').classList.add('hidden');
+    document.getElementById('btn-loop-save').classList.add('hidden');
+    document.getElementById('loop-label').textContent = '';
+    document.getElementById('saved-loops').value = '';
+    resetSelection();
+    _updateSectionPracticeHighlight(_audioTime());
+    if (hadLoop && emitTransportEvent && typeof window !== 'undefined') {
+        window.feedBack?.playback?.transportEvent?.('loop-cleared', {
+            requesterId: 'core.loop',
+            reason: 'app loop cleared',
+            loop: { enabled: false, state: 'inactive' },
+        });
+    }
+}
+
+// Resync #saved-loops + #btn-loop-delete with the currently-active
+// loopA/loopB. Used by both setLoop's success path (so plugin-driven
+// loops show up correctly in the dropdown) and loadSavedLoop's
+// failure path (so a cancelled selection reverts to the still-active
+// loop). Without this sync, deleteSelectedLoop could target a stale
+// option that doesn't match the active loop.
+function _syncSavedLoopSelection() {
+    const sel = document.getElementById('saved-loops');
+    const delBtn = document.getElementById('btn-loop-delete');
+    if (!sel || !delBtn) return;
+    let selected = '';
+    if (loopA !== null && loopB !== null) {
+        for (const opt of sel.options) {
+            if (Number(opt.dataset.start) === loopA && Number(opt.dataset.end) === loopB) {
+                selected = opt.value;
+                break;
+            }
+        }
+    }
+    sel.value = selected;
+    delBtn.classList.toggle('hidden', !selected);
+}
+
+// Programmatically set both loop endpoints and seek to A. The dropdown
+// path (loadSavedLoop) and the plugin-API path (window.feedBack.setLoop)
+// both funnel through here so the UI state stays canonical regardless of
+// who triggered the loop.
+//
+// Returns true if the seek landed at A and the loop is now active;
+// returns false if the seek was cancelled by teardown or landed off-target
+// (JUCE clamp / HTML5 snap > 50ms from A). On false, loopA/loopB are NOT
+// committed and the UI is not painted — the prior loop (if any) stays
+// active. Throws on invalid inputs.
+export async function setLoop(a, b, options) {
+    const { emitTransportEvent = true, skipSectionSync = false, commitGuard = null } = options || {};
+    const aNum = Number(a);
+    const bNum = Number(b);
+    if (!Number.isFinite(aNum) || !Number.isFinite(bNum) || bNum <= aNum) {
+        throw new Error(`setLoop: requires finite a and b with b > a (got a=${a}, b=${b})`);
+    }
+    // Don't arm loopA/loopB before the seek lands — the 60Hz tick's wrap
+    // detector (`ct >= loopB`) would trigger startCountIn against
+    // half-applied state.
+    const r = await _audioSeek(aNum, 'loop-set');
+    if (!r.completed || Math.abs(r.to - aNum) > 0.05) return false;
+    // Caller-owned staleness gate, re-checked after the awaited seek and before
+    // we commit loopA/loopB. practiceSection() passes this so a superseded retry
+    // (newer section click, mode turned off, or song/arrangement teardown that
+    // happened during the seek) does not arm a stale loop. Returning false here
+    // leaves the prior loop (if any) untouched, same as the off-target path.
+    if (typeof commitGuard === 'function' && !commitGuard()) return false;
+    loopA = aNum;
+    loopB = bNum;
+    // A direct (non-practice) loop set supersedes any in-flight practiceSection
+    // retry; practiceSection passes skipSectionSync and is exempt so it doesn't
+    // cancel itself.
+    if (!skipSectionSync) _loopMutationGen++;
+    document.getElementById('btn-loop-a').className = 'px-3 py-1.5 bg-green-900/50 rounded-lg text-xs text-green-300 transition';
+    document.getElementById('btn-loop-b').className = 'px-3 py-1.5 bg-green-900/50 rounded-lg text-xs text-green-300 transition';
+    updateLoopUI();
+    // Sync the saved-loops dropdown so a plugin-driven setLoop call
+    // surfaces the matching saved option (and Delete button) — otherwise
+    // the dropdown can stay on a stale selection and deleteSelectedLoop
+    // would target the wrong record.
+    _syncSavedLoopSelection();
+    // practiceSection() passes skipSectionSync: it sets its own section state
+    // under a request-gen guard, so the shared setLoop path must NOT re-sync
+    // here — otherwise a stale (superseded / mode-off) practiceSection retry
+    // that lands inside setLoop would re-arm the loop and flip the mode back on
+    // before the caller's gen check can bail. Direct callers (Saved Loops,
+    // window.feedBack.setLoop) still sync so their chip selection tracks.
+    if (!skipSectionSync && typeof _syncSectionPracticeFromLoop === 'function') {
+        _syncSectionPracticeFromLoop();
+    }
+    if (emitTransportEvent && typeof window !== 'undefined') {
+        window.feedBack?.playback?.transportEvent?.('loop-set', { requesterId: 'core.loop', loopA, loopB, loop: { startTime: loopA, endTime: loopB, enabled: true, state: 'active' } });
+    }
+    return true;
+}
+
+export function updateLoopUI() {
+    const label = document.getElementById('loop-label');
+    const hasLoop = loopA !== null && loopB !== null;
+    if (hasLoop) {
+        label.textContent = `${formatTime(loopA)} → ${formatTime(loopB)}`;
+        document.getElementById('btn-loop-clear').classList.remove('hidden');
+        document.getElementById('btn-loop-save').classList.remove('hidden');
+    } else if (loopA !== null) {
+        label.textContent = `${formatTime(loopA)} → ?`;
+        document.getElementById('btn-loop-clear').classList.add('hidden');
+        document.getElementById('btn-loop-save').classList.add('hidden');
+    } else {
+        label.textContent = '';
+    }
+    host._updateEditRegionBtn();
+}
+
+export async function loadSavedLoops() {
+    const sel = document.getElementById('saved-loops');
+    const delBtn = document.getElementById('btn-loop-delete');
+    if (!host.currentFilename()) { sel.classList.add('hidden'); delBtn.classList.add('hidden'); return; }
+
+    const resp = await fetch(`/api/loops?filename=${encodeURIComponent(decodeURIComponent(host.currentFilename()))}`);
+    const loops = await resp.json();
+
+    sel.innerHTML = '<option value="">Saved Loops</option>';
+    for (const l of loops) {
+        sel.innerHTML += `<option value="${l.id}" data-start="${l.start}" data-end="${l.end}">${esc(l.name)} (${formatTime(l.start)}→${formatTime(l.end)})</option>`;
+    }
+    if (loops.length > 0) {
+        sel.classList.remove('hidden');
+    } else {
+        sel.classList.add('hidden');
+    }
+    delBtn.classList.add('hidden');
+}
+
+export async function loadSavedLoop(loopId) {
+    const sel = document.getElementById('saved-loops');
+    const opt = sel.selectedOptions[0];
+    const delBtn = document.getElementById('btn-loop-delete');
+    if (!loopId || !opt?.dataset.start) {
+        delBtn.classList.add('hidden');
+        return;
+    }
+    let ok = false;
+    try {
+        // Pass raw strings — setLoop's Number() coercion is stricter than
+        // parseFloat (rejects "12abc") so malformed dataset values throw
+        // and fall into the catch instead of silently truncating.
+        ok = await setLoop(opt.dataset.start, opt.dataset.end);
+    } catch (err) {
+        // Malformed dataset (server returned bad data): treat the same as
+        // a failed seek so the dropdown resyncs and we don't propagate an
+        // uncaught rejection out of the onchange handler.
+        console.warn('[loadSavedLoop] setLoop threw:', err);
+        ok = false;
+    }
+    if (!ok) {
+        // Seek aborted, landed off-target, or input was malformed.
+        // Resync the dropdown with the still-active loop so the UI
+        // doesn't lie about which loop is loaded.
+        _syncSavedLoopSelection();
+        return;
+    }
+    // Success path: setLoop already called _syncSavedLoopSelection,
+    // which surfaces the delete button when the new loop matches a
+    // saved option (which the dropdown selection guarantees here).
+}
+
+export async function saveCurrentLoop() {
+    if (loopA === null || loopB === null || !host.currentFilename()) return;
+    const name = await uiPrompt({ title: 'Save Loop', label: 'Loop name', value: 'Loop', okLabel: 'Save' });
+    if (name === null) return;          // cancelled
+    const finalName = name.trim() || 'Loop';   // never persist an empty name
+    await fetch('/api/loops', {
+        method: 'POST',
+        headers: { 'Content-Type': 'application/json' },
+        body: JSON.stringify({
+            filename: decodeURIComponent(host.currentFilename()),
+            name: finalName,
+            start: loopA,
+            end: loopB,
+        }),
+    });
+    await loadSavedLoops();
+    document.getElementById('btn-loop-save').classList.add('hidden');
+}
+
+export async function deleteSelectedLoop() {
+    const sel = document.getElementById('saved-loops');
+    const loopId = sel.value;
+    if (!loopId) return;
+    await fetch(`/api/loops/${loopId}`, { method: 'DELETE' });
+    clearLoop();
+    await loadSavedLoops();
+}

static/js/player-controls.js

@@ -0,0 +1,229 @@
+// Player controls — the speed and mastery sliders, and the four playback preference
+// reads (autoplay-exit, up-next, countdown-before-song, confirm-exit).
+//
+// The fourth slice out of app.js's strongly-connected core, and by far the easiest:
+// ONE hook and NO shared mutable state. It is here because these three groups are the
+// same surface (the controls under the highway) and all three reach the same helper.
+//
+// The preference reads are one-line localStorage lookups that half of app.js consults
+// before deciding whether to auto-start, show the Up Next pill, run a count-in, or
+// confirm on exit. They travel with the controls that set them.
+//
+// See ./host.js: reading an unwired hook THROWS, and tests/js/host_contract.test.js
+// fails CI if the hooks used here and the hooks app.js wires ever drift apart.
+import { audio } from './audio-el.js';
+import { host } from './host.js';
+
+// ── Autoplay & auto-exit (global option, default ON) ──────────────────
+// One toggle (`autoplayExit` in localStorage) that (a) auto-starts a song
+// once it's ready and (b) returns to the launching menu when the song
+// ends. Absence of the key means enabled. The behaviour lives in core
+// (app.js, shared by the v3 + classic UIs); the end-of-song *score*
+// screen, when present, is a plugin and hooks the contract below.
+export function _autoplayExitEnabled() {
+    try { return localStorage.getItem('autoplayExit') !== '0'; } catch (_) { return true; }
+}
+
+// ── "Up Next" pill (global option, default ON) ────────────────────────
+// Gates the v3 player chrome's persistent upcoming-section pill
+// (#v3-upnext, driven by player-chrome.js's updateUpNext). Client-only
+// localStorage pref (`showUpNext`); absence of the key means enabled.
+// player-chrome.js reads window.feedBack.showUpNext each tick and hides
+// the pill when off.
+export function _showUpNextEnabled() {
+    try { return localStorage.getItem('showUpNext') !== '0'; } catch (_) { return true; }
+}
+
+// "Countdown before song" (Gameplay tab). Mirrored to localStorage by
+// loadSettings so the song-start path can read it synchronously here — no
+// async /api/settings fetch on the play hot path. Defaults off.
+export function _countdownBeforeSongEnabled() {
+    try { return localStorage.getItem('countdownBeforeSong') === '1'; } catch (_) { return false; }
+}
+
+export function _curPlaybackSpeed() {
+    try {
+        return window._juceMode
+            ? ((window.jucePlayer && window.jucePlayer._speed) || 1)
+            : (document.getElementById('audio')?.playbackRate || 1);
+    } catch (_) { return 1; }
+}
+
+// ── "Ask before leaving a song" (Gameplay tab, default OFF) ────────────────
+// Client-only localStorage pref (`confirmExitSong`); absence = OFF. When ON, a
+// *user-initiated* exit (Escape, or the player ✕) opens a small confirm instead
+// of leaving immediately. Auto-exit on song-end and a results screen's own
+// Close never prompt — they call closeCurrentSong() directly, which stays the
+// unguarded actual-exit.
+export function _exitConfirmEnabled() {
+    try { return localStorage.getItem('confirmExitSong') === '1'; } catch (_) { return false; }
+}
+
+const SPEED_PRESET_PCTS = [100, 90, 80, 75, 70, 60, 50];
+const SPEED_SNAP_THRESHOLD = 0.02;
+let _speedPresetsWired = false;
+
+function _speedPresetPctFromActive(activePctOrRate) {
+    if (!Number.isFinite(activePctOrRate)) return null;
+    const rate = activePctOrRate <= 1.5 ? activePctOrRate : activePctOrRate / 100;
+    for (const pct of SPEED_PRESET_PCTS) {
+        if (Math.abs(rate - pct / 100) <= SPEED_SNAP_THRESHOLD) return pct;
+    }
+    return null;
+}
+
+function _updateSpeedPresetButtons(activePctOrRate) {
+    const wrap = document.getElementById('speed-presets');
+    if (!wrap) return;
+    const target = _speedPresetPctFromActive(activePctOrRate);
+    for (const btn of wrap.querySelectorAll('[data-speed-preset]')) {
+        const pct = Number(btn.dataset.speedPreset);
+        btn.classList.toggle('v3-speed-preset-active', target !== null && pct === target);
+    }
+}
+
+export function applySpeedPreset(percent) {
+    const slider = document.getElementById('speed-slider');
+    if (!slider) return;
+    const pct = Math.max(
+        Number(slider.min) || 15,
+        Math.min(Number(slider.max) || 150, Number(percent)),
+    );
+    if (!Number.isFinite(pct)) return;
+    slider.value = String(pct);
+    host.handleSliderInput(slider);
+    slider.dispatchEvent(new Event('input', { bubbles: true }));
+}
+
+export function _wireSpeedPresetsOnce() {
+    if (_speedPresetsWired) return;
+    const presets = document.getElementById('speed-presets');
+    if (!presets) return;
+    _speedPresetsWired = true;
+    presets.addEventListener('click', (e) => {
+        const btn = e.target.closest('[data-speed-preset]');
+        if (!btn) return;
+        applySpeedPreset(Number(btn.dataset.speedPreset));
+    });
+}
+
+export function setSpeed(v) {
+    const speedSlider = document.getElementById('speed-slider');
+    const rate = Number(v);
+    if (!Number.isFinite(rate)) {
+        return;
+    }
+    if (window._juceMode) {
+        window.jucePlayer?.setRate(rate);
+        const juceAudio = window.feedBackDesktop?.audio;
+        Promise.resolve()
+            .then(() => juceAudio?.setBackingSpeed(rate))
+            // Match the HTML5 path: preserve pitch on the JUCE backing track too.
+            // Optional-chained call is a no-op on desktop builds that predate
+            // setBackingPreservePitch, so this is safe to ship unconditionally.
+            .then(() => juceAudio?.setBackingPreservePitch?.(true))
+            .catch(err => console.warn('[setSpeed] backing speed/preserve-pitch failed:', err));
+    } else {
+        audio.playbackRate = rate;
+    }
+    const speedLabel = document.getElementById('speed-label');
+    if (speedLabel) speedLabel.textContent = rate.toFixed(2) + 'x';
+    host.handleSliderInput(speedSlider);
+    _updateSpeedPresetButtons(rate);
+}
+
+export function _resetPlaybackSpeedForNewSong() {
+    // Reset the *actual* playback rate to 1x, not just the visible slider/label
+    // (feedBack#615). The HTML5 <audio> element and the desktop JUCE/backing
+    // engine each retain their own rate, and which one drives the next song
+    // isn't decided until later in the load, so reset all paths unconditionally.
+    // Every setter is idempotent and optional-chained, so this is safe in web
+    // and desktop builds alike — no need to branch on window._juceMode.
+    const speedSlider = document.getElementById('speed-slider');
+    if (speedSlider) speedSlider.value = 100;
+    audio.playbackRate = 1;
+    window.jucePlayer?.setRate?.(1);
+    const juceAudio = window.feedBackDesktop?.audio;
+    Promise.resolve()
+        .then(() => juceAudio?.setBackingSpeed?.(1))
+        .then(() => juceAudio?.setBackingPreservePitch?.(true))
+        .catch(err => console.warn('[resetSpeed] backing speed/preserve-pitch failed:', err));
+    // Mirror setSpeed's UI side-effects (label text + slider fill styling).
+    const speedLabel = document.getElementById('speed-label');
+    if (speedLabel) speedLabel.textContent = (1).toFixed(2) + 'x';
+    host.handleSliderInput(speedSlider);
+    _updateSpeedPresetButtons(100);
+}
+// Master-difficulty slider (feedBack#48). Persists partial via
+// /api/settings — the POST handler merges only the keys present, so
+// this fire-and-forget call doesn't clobber dlc_dir or other settings.
+//
+// Debounced trailing-edge (300ms) so dragging the slider — which fires
+// oninput per pixel — doesn't flood the server with concurrent writes
+// to config.json. highway.setMastery() still fires every oninput so
+// the chart re-filters in real time; only disk persistence waits.
+let _masteryPersistTimer = null;
+function _persistMastery(pct) {
+    if (_masteryPersistTimer) clearTimeout(_masteryPersistTimer);
+    _masteryPersistTimer = setTimeout(() => {
+        _masteryPersistTimer = null;
+        fetch('/api/settings', {
+            method: 'POST',
+            headers: { 'Content-Type': 'application/json' },
+            body: JSON.stringify({ master_difficulty: pct }),
+        }).catch(() => { /* best-effort — next setMastery() will retry */ });
+    }, 300);
+}
+export function setMastery(v) {
+    _applyMastery(v);
+}
+// Shared mastery applier. Master difficulty has two controls that write the
+// same master_difficulty key: the player-popover slider (#mastery-slider) and
+// the Gameplay-tab "Note highway speed" slider (#setting-highway-speed). Route
+// both — and loadSettings' hydration — through here so their positions,
+// labels, and track fills stay in sync regardless of which the user touches,
+// plus the live highway re-filter and the debounced persist. All element reads
+// are null-guarded since either control may be absent (follower window, or the
+// settings markup not yet rendered).
+export function _applyMastery(v, opts = {}) {
+    // Guard + clamp: v might be a slider string, a programmatic call from a
+    // plugin, or a restored settings value with a bad shape. Don't let NaN
+    // reach a label (would show "NaN%") or the POST.
+    const parsed = parseInt(v, 10);
+    if (!Number.isFinite(parsed)) return;
+    const pct = Math.max(0, Math.min(100, parsed));
+    const popLabel = document.getElementById('mastery-label');
+    if (popLabel) popLabel.textContent = pct + '%';
+    const popSlider = document.getElementById('mastery-slider');
+    if (popSlider) {
+        if (String(popSlider.value) !== String(pct)) popSlider.value = pct;
+        host.handleSliderInput(popSlider);
+    }
+    const setSlider = document.getElementById('setting-highway-speed');
+    if (setSlider) {
+        if (String(setSlider.value) !== String(pct)) setSlider.value = pct;
+        host.handleSliderInput(setSlider);
+    }
+    // The Gameplay-tab label markup appends a literal "%" after this span
+    // (matching the av-offset "ms" pattern), so write the number alone here —
+    // unlike #mastery-label above, whose markup carries no trailing unit.
+    const setLabel = document.getElementById('setting-highway-speed-val');
+    if (setLabel) setLabel.textContent = pct;
+    highway.setMastery(pct / 100);
+    if (!opts.skipPersist) _persistMastery(pct);
+}
+// Reflect phrase-data availability on the slider after every `ready`.
+// The server omits the `phrases` message entirely for single-level
+// sources (GP imports, legacy sloppak), so hasPhraseData() is the
+// right signal to enable/disable the slider.
+export function _applyMasteryAvailability(hasPhraseData) {
+    const slider = document.getElementById('mastery-slider');
+    if (!slider) return;
+    if (hasPhraseData) {
+        slider.disabled = false;
+        slider.title = 'Master difficulty — low = simpler chart, high = full';
+    } else {
+        slider.disabled = true;
+        slider.title = 'Source chart has a single difficulty level — slider disabled';
+    }
+}

static/js/player-state.js

@@ -0,0 +1,42 @@
+// Shared, MUTABLE player state.
+//
+// WHY A CONTAINER AND NOT PLAIN EXPORTS. An imported binding is read-only. Every
+// slice carved out of app.js so far has only ever READ the state it shares
+// (loopA/loopB, _audioSeekGen, currentFilename), so a getter hook was enough and no
+// container was needed. That runs out here: count-in genuinely WRITES `isPlaying`
+// (it starts and stops playback) and `lastAudioTime`. `import { isPlaying }` then
+// `isPlaying = true` throws — the binding cannot be assigned to.
+//
+// So the state moves onto an object. `S.isPlaying = true` is a property write, which
+// works from any module holding the same `S`. This is the same shape the stems,
+// studio, and editor migrations converged on.
+//
+// It is deliberately SMALL. app.js has ~104 top-level `let` scalars; lifting all of
+// them would be a ~977-site rewrite for no benefit, since most are private to one
+// cluster and travel with it. Only the ones a carved module must WRITE belong here.
+// Add to it when a carve actually needs it, not before.
+//
+// NB app.js's own 71 reference sites were rewritten mechanically — but from the AST,
+// not by text substitution. Of 100 textual occurrences of these two names, only 71
+// resolve to the module binding: 22 are member accesses (`someObj.isPlaying`), 4 are
+// the local parameter of setPlayButtonState(isPlaying), one is an object key, and two
+// are shorthand properties (`{ isPlaying }`) that must become `{ isPlaying: S.isPlaying }`.
+// A blind find-and-replace corrupts all 29.
+export const S = {
+    /** Is the transport running? Written by playback, count-in, and the JUCE shims. */
+    isPlaying: false,
+
+    /**
+     * The last audio position we saw, in seconds. Used to detect a seek that did not
+     * land where it was asked to (JUCE can clamp; HTML5 can round).
+     */
+    lastAudioTime: 0,
+
+    /**
+     * A resume request armed by playSong({ resume }) and consumed on song:ready.
+     * Written by app.js (playSong, and the song:ready listener that consumes it) and
+     * read by the resume-session module — so, like the two above, it cannot be a plain
+     * export.
+     */
+    pendingResume: null,
+};

static/js/plugin-loader.js

@@ -654,7 +654,8 @@ export async function loadPlugins() {
                         // of a cached copy keyed only by path (matches the art
                         // URL ?v=mtime convention elsewhere in this file).
                         const v = encodeURIComponent(wantedVersion);
-                        script.src = `/api/plugins/${plugin.id}/screen.js${v ? `?v=${v}` : ''}`;
+                        const query = v ? `?v=${v}` : '';
+                        script.src = _pluginScriptUrl(plugin, wantedVersion, query);
                         // Module-migration (R0): a migrated plugin declares
                         // scriptType:"module" and its screen.js is `import
                         // './src/main.js'`. A <script type="module"> fires load
@@ -802,3 +803,112 @@ export async function bootstrapPluginsAndUi() {
     _streamPluginStartup();
     return plugins;
 }
+
+
+// ── Plugin updates ──────────────────────────────────────────────────────
+// The Settings-screen "Check for updates" / "Update" buttons. Carved out of
+// app.js (R3a) into the loader rather than a module of their own: this is plugin
+// MANAGEMENT, it belongs with the code that loads them. Both are inline handlers,
+// so app.js re-exposes them on window.
+
+export async function checkPluginUpdates() {
+    const btn = document.getElementById('btn-check-updates');
+    const status = document.getElementById('updates-status');
+    const list = document.getElementById('plugin-updates-list');
+    btn.disabled = true;
+    btn.textContent = 'Checking...';
+    status.textContent = '';
+    list.innerHTML = '';
+    try {
+        const resp = await fetch('/api/plugins/updates');
+        const data = await resp.json();
+        const updates = data.updates || {};
+        const keys = Object.keys(updates);
+        if (keys.length === 0) {
+            status.textContent = 'All plugins are up to date.';
+        } else {
+            status.textContent = `${keys.length} update${keys.length > 1 ? 's' : ''} available`;
+            for (const id of keys) {
+                const u = updates[id];
+                const row = document.createElement('div');
+                row.className = 'flex items-center gap-3 bg-dark-700 rounded-lg px-4 py-2';
+                row.innerHTML = `
+                    <span class="text-sm text-gray-300 flex-1">${u.name} <span class="text-xs text-gray-500">(${u.behind} commit${u.behind > 1 ? 's' : ''} behind — ${u.local} → ${u.remote})</span></span>
+                    <button onclick="updatePlugin('${id}', this)" class="bg-accent/20 hover:bg-accent/30 text-accent-light px-3 py-1 rounded-lg text-xs transition">Update</button>`;
+                list.appendChild(row);
+            }
+        }
+    } catch (e) {
+        status.textContent = 'Failed to check for updates.';
+    }
+    btn.disabled = false;
+    btn.textContent = 'Check for Updates';
+}
+
+// ── Module re-evaluation (#879) ─────────────────────────────────────────────
+//
+// ES modules are evaluated ONCE PER URL PER DOCUMENT. Re-inserting a
+// <script type="module"> whose src the module map has already seen fires `load` but
+// does NOT re-run the body. So a ROLLBACK — reloading a version already evaluated
+// this session — silently kept the OLD module live, while onload fired and
+// loadedScripts recorded the rollback as applied. A no-op that reported success.
+// (Upgrades were fine: a new version means a new ?v=, hence a new URL.)
+//
+// Busting the ENTRY url alone does NOT fix it. A module plugin's screen.js is a
+// one-line `import './src/main.js'`, and a relative specifier resolves against the
+// base URL WITH THE QUERY STRING DROPPED — so ?v= never reaches the graph, and
+// src/main.js (where the plugin actually lives) stays cached no matter what we hang
+// off screen.js.
+//
+// So the token goes in the PATH. From /api/plugins/x/g/7/screen.js, './src/main.js'
+// resolves to /api/plugins/x/g/7/src/main.js — every relative import in the graph
+// inherits it, at every depth, with no import-specifier rewriting (which could not
+// see `import(expr)` anyway). The server ignores the token and serves identical
+// bytes.
+//
+// ─── AND THE UPGRADE PATH WAS BROKEN TOO ────────────────────────────────────
+//
+// #879 says "upgrades are fine — a new version yields a new URL". That is true of
+// screen.js and FALSE of the plugin. Driving a real browser through
+// install(1.0.0) -> upgrade(1.1.0) -> rollback(1.0.0) and counting evaluations of
+// src/main.js gives ONE. Not two, not three: ONE. The upgrade re-evaluates the
+// one-line screen.js shim at its new ?v= URL, that shim imports './src/main.js',
+// that resolves to the same URL as before, and the module map hands back the
+// ALREADY-EVALUATED v1.0.0 module. The plugin's actual code never re-ran.
+//
+// So the generation token is not a rollback special case. EVERY re-load of a module
+// plugin needs it — the key is the plugin id, NOT id@version. Only the first load of
+// a given plugin in this document takes the stable URL, which is what keeps the
+// ETag/304 live-edit contract the R0 rails depend on.
+const _evaluatedModules = new Set();   // plugin ids whose module graph is live in this document
+let _moduleReloadSeq = 0;
+
+function _pluginScriptUrl(plugin, wantedVersion, query) {
+    const base = `/api/plugins/${plugin.id}/screen.js${query}`;
+    if (plugin.script_type !== 'module') return base;   // classic scripts always re-run
+    if (!_evaluatedModules.has(plugin.id)) {
+        _evaluatedModules.add(plugin.id);
+        return base;                                    // first load: stable URL, 304-able
+    }
+    // Re-load of a module plugin — upgrade OR rollback. Its graph is already in the
+    // module map, so it needs an entirely fresh path or nothing below screen.js re-runs.
+    return `/api/plugins/${plugin.id}/g/${++_moduleReloadSeq}/screen.js${query}`;
+}
+
+export async function updatePlugin(pluginId, btn) {
+    btn.disabled = true;
+    btn.textContent = 'Updating...';
+    try {
+        const resp = await fetch(`/api/plugins/${pluginId}/update`, { method: 'POST' });
+        const data = await resp.json();
+        if (data.ok) {
+            btn.textContent = 'Updated — restart to apply';
+            btn.className = 'bg-green-900/30 text-green-400 px-3 py-1 rounded-lg text-xs';
+        } else {
+            btn.textContent = 'Failed';
+            btn.title = data.error || '';
+        }
+    } catch (e) {
+        btn.textContent = 'Error';
+    }
+}

static/js/resume-session.js

@@ -0,0 +1,157 @@
+// Resume last session — the snapshot taken when you leave a song, and the pill that
+// offers it back.
+//
+// The fifth slice out of app.js's strongly-connected core. Small and self-contained:
+// ONE hook (playSong) plus a currentFilename getter.
+//
+// The armed resume request itself lives on the shared container as S.pendingResume,
+// not here, because app.js WRITES it — playSong({ resume }) arms it and the song:ready
+// listener consumes it — while this module reads it. An imported binding is read-only,
+// so shared mutable state has to live on the container. Same reason isPlaying does.
+//
+// See ./host.js: reading an unwired hook THROWS, and tests/js/host_contract.test.js
+// fails CI if the hooks used here and the hooks app.js wires ever drift apart.
+import { host } from './host.js';
+import { _curPlaybackSpeed } from './player-controls.js';
+import { S } from './player-state.js';
+
+// ── Resume last session ────────────────────────────────────────────────────
+// Leaving a song snapshots where you were — song, arrangement, position, and
+// speed — so an exit (especially an accidental one, now that Escape reliably
+// leaves regardless of focus) is recoverable instead of restarting from bar 1.
+// The snapshot is offered back through a non-blocking "Resume" pill; it never
+// gates, blocks, or auto-acts. Cleared on natural song-end and once consumed.
+// (This is the player-session slice; the broader nav/state-resume work — e.g.
+// returning to a song after wandering into Settings → Tone Builder — is a
+// separate, larger track.)
+const _RESUME_KEY = 'feedBack.resumeSession';
+const _RESUME_MAX_AGE_MS = 24 * 60 * 60 * 1000;   // a day-old snapshot is stale
+const _RESUME_MIN_POSITION_S = 3;                  // ignore barely-started songs
+const _RESUME_END_GUARD_S = 5;                      // ignore basically-finished songs
+let _resumePillDismissed = false;                   // per-session: user waved off the current snapshot
+
+// Snapshot the live session. Called from showScreen()'s teardown before
+// highway.stop()/audio unload, while getSongInfo() + position are still valid.
+export function _snapshotResumeSession(position) {
+    try {
+        if (!host.currentFilename()) return;
+        const si = (window.highway && typeof highway.getSongInfo === 'function')
+            ? (highway.getSongInfo() || {}) : {};
+        const dur = Number(si.duration) || 0;
+        const pos = Number(position) || 0;
+        // Only worth resuming a song you were genuinely mid-way through — not a
+        // glance at the first seconds, and not one that already basically ended.
+        if (pos < _RESUME_MIN_POSITION_S) { _clearResumeSession(); return; }
+        if (dur && pos > dur - _RESUME_END_GUARD_S) { _clearResumeSession(); return; }
+        const snap = {
+            f: host.currentFilename(),
+            a: (typeof si.arrangement_index === 'number' && si.arrangement_index >= 0)
+                ? si.arrangement_index : undefined,
+            t: pos,
+            sp: _curPlaybackSpeed(),
+            title: si.title || '',
+            artist: si.artist || '',
+            ts: Date.now(),
+        };
+        localStorage.setItem(_RESUME_KEY, JSON.stringify(snap));
+        // A fresh snapshot earns one offer — undo any earlier dismissal.
+        _resumePillDismissed = false;
+    } catch (_) { /* storage unavailable — resume is best-effort */ }
+}
+
+export function _readResumeSession() {
+    try {
+        const raw = localStorage.getItem(_RESUME_KEY);
+        if (!raw) return null;
+        const snap = JSON.parse(raw);
+        if (!snap || !snap.f || !(Number(snap.t) > 0)) return null;
+        if (!snap.ts || Date.now() - snap.ts > _RESUME_MAX_AGE_MS) { _clearResumeSession(); return null; }
+        return snap;
+    } catch (_) { return null; }
+}
+
+export function _clearResumeSession() {
+    try { localStorage.removeItem(_RESUME_KEY); } catch (_) {}
+}
+
+// Re-enter the snapshotted song and restore arrangement + position + speed.
+export async function resumeLastSession() {
+    const snap = _readResumeSession();
+    if (!snap) { _hideResumePill(); return false; }
+    _hideResumePill();
+    try {
+        await host.playSong(snap.f, snap.a, {
+            resume: { position: Number(snap.t) || 0, speed: Number(snap.sp) || 1 },
+        });
+    } catch (err) {
+        // A transient load/connect failure must not strand the user: keep the
+        // snapshot so the pill can re-offer it on the next non-player screen,
+        // rather than consuming the only copy before the song actually loaded.
+        console.warn('[app] resume failed to load; keeping snapshot:', err);
+        S.pendingResume = null;
+        return false;
+    }
+    _clearResumeSession();   // consumed only after a successful load
+    return true;
+}
+
+// ── Resume pill (non-blocking "continue where you left off") ────────────────
+// Self-contained, inline-styled, body-appended so it works identically in the
+// classic (v2) and v3 shells with no Tailwind rebuild. It only ever appears off
+// the player screen, never blocks, and a dismiss forgets the current snapshot
+// for the session.
+export function _hideResumePill() {
+    const el = document.getElementById('fb-resume-pill');
+    if (el) el.remove();
+}
+
+export function _maybeShowResumePill() {
+    const active = document.querySelector('.screen.active');
+    if (active && active.id === 'player') { _hideResumePill(); return; }
+    if (_resumePillDismissed) return;
+    const snap = _readResumeSession();
+    if (!snap) { _hideResumePill(); return; }
+    if (document.getElementById('fb-resume-pill')) return;   // already shown
+
+    const label = (snap.title || decodeURIComponent(snap.f || 'your last song')).toString();
+    const pill = document.createElement('div');
+    pill.id = 'fb-resume-pill';
+    pill.setAttribute('role', 'status');
+    pill.style.cssText = [
+        'position:fixed', 'left:16px', 'bottom:16px', 'z-index:120',
+        'display:flex', 'align-items:center', 'gap:10px',
+        'max-width:min(90vw,360px)', 'padding:10px 12px',
+        'background:rgba(17,24,39,0.96)', 'color:#e5e7eb',
+        'border:1px solid rgba(148,163,184,0.25)', 'border-radius:10px',
+        'box-shadow:0 6px 24px rgba(0,0,0,0.4)',
+        'font:13px/1.3 system-ui,-apple-system,"Segoe UI",Roboto,sans-serif',
+    ].join(';');
+
+    const text = document.createElement('div');
+    text.style.cssText = 'flex:1;min-width:0';
+    const t1 = document.createElement('div');
+    t1.textContent = 'Resume practice';
+    t1.style.cssText = 'font-weight:600;color:#fff';
+    const t2 = document.createElement('div');
+    t2.textContent = label;
+    t2.style.cssText = 'opacity:0.7;white-space:nowrap;overflow:hidden;text-overflow:ellipsis';
+    text.appendChild(t1); text.appendChild(t2);
+
+    const resumeBtn = document.createElement('button');
+    resumeBtn.type = 'button';
+    resumeBtn.textContent = 'Resume ▸';
+    resumeBtn.style.cssText = 'flex:none;padding:6px 10px;border:0;border-radius:7px;background:#4080e0;color:#fff;font-weight:600;cursor:pointer';
+    resumeBtn.addEventListener('click', () => { resumeLastSession(); });
+
+    const dismissBtn = document.createElement('button');
+    dismissBtn.type = 'button';
+    dismissBtn.setAttribute('aria-label', 'Dismiss');
+    dismissBtn.textContent = '✕';
+    dismissBtn.style.cssText = 'flex:none;padding:4px 6px;border:0;border-radius:7px;background:transparent;color:#9ca3af;cursor:pointer;font-size:14px';
+    dismissBtn.addEventListener('click', () => { _resumePillDismissed = true; _hideResumePill(); });
+
+    pill.appendChild(text);
+    pill.appendChild(resumeBtn);
+    pill.appendChild(dismissBtn);
+    (document.body || document.documentElement).appendChild(pill);
+}

static/js/settings-io.js

@@ -0,0 +1,155 @@
+// Settings backup — the export / import bundle.
+//
+// Carved verbatim out of static/app.js (R3a). A LEAF module: imports nothing.
+//
+// Two entry points, both inline handlers on the Settings screen, so app.js keeps
+// re-exposing them on window. The import is two-phase (server first, atomic; then
+// a best-effort localStorage merge) — the rationale comment below is the contract
+// and moved with the code.
+
+//
+// Bundles server config + every localStorage key + opted-in plugin server
+// files into a single JSON file.
+//
+// Apply semantics — phased, NOT all-or-nothing across the two stores:
+//   1. Server first (/api/settings/import). Phase-1 validation guards
+//      the whole bundle; phase-2 disk commit is per-file but ordered
+//      so a mid-apply failure surfaces a `partial` field. A server
+//      failure short-circuits before any localStorage write, so the
+//      browser side stays untouched on validation refusals.
+//   2. localStorage second, only after the server returns ok. Applied
+//      as a MERGE (no clear): bundled keys overwrite, locally-present
+//      keys absent from the bundle are preserved (so a plugin
+//      installed after the export keeps its first-run defaults).
+//      A localStorage exception here (quota / private mode) is
+//      surfaced verbatim — server state is already committed and we
+//      don't pretend the import was clean.
+//
+// In short: the server side is atomic in phase 1 and surface-partial in
+// phase 2; the localStorage side is best-effort merge after server
+// success. Failures are reported, never silenced.
+
+export async function exportSettings() {
+    const status = document.getElementById('backup-status');
+    status.textContent = 'Exporting...';
+    try {
+        const resp = await fetch('/api/settings/export');
+        if (!resp.ok) {
+            status.textContent = `Export failed (HTTP ${resp.status})`;
+            return;
+        }
+        const bundle = await resp.json();
+        // Layer in the browser's localStorage. Use the standard Storage
+        // iteration API (length + key(i)) rather than Object.keys —
+        // Object.keys on a Storage instance is not deterministic across
+        // browsers and can both miss entries and include non-entry
+        // properties depending on the implementation. Keys are preserved
+        // verbatim as strings; that's how localStorage stores them, and
+        // round-trip fidelity matters more than re-typing values that
+        // were never typed in the first place.
+        const localStorageData = {};
+        for (let i = 0; i < localStorage.length; i++) {
+            const key = localStorage.key(i);
+            if (key === null) continue;
+            const value = localStorage.getItem(key);
+            if (value !== null) localStorageData[key] = value;
+        }
+        bundle.local_storage = localStorageData;
+
+        // Trigger download via blob + temporary <a download>. We honor the
+        // server's Content-Disposition filename when present, otherwise
+        // fall back to a date-stamped default.
+        let filename = 'feedBack-settings.json';
+        const disposition = resp.headers.get('Content-Disposition');
+        if (disposition) {
+            const match = /filename="([^"]+)"/.exec(disposition);
+            if (match) filename = match[1];
+        }
+        const blob = new Blob([JSON.stringify(bundle, null, 2)], { type: 'application/json' });
+        const url = URL.createObjectURL(blob);
+        const a = document.createElement('a');
+        a.href = url;
+        a.download = filename;
+        document.body.appendChild(a);
+        a.click();
+        document.body.removeChild(a);
+        URL.revokeObjectURL(url);
+        status.textContent = `Exported ${filename}`;
+    } catch (e) {
+        status.textContent = `Export failed: ${e.message}`;
+    }
+}
+
+export async function importSettings(file) {
+    if (!file) return;
+    const status = document.getElementById('backup-status');
+    if (!confirm('Import will overwrite settings present in the bundle (server config, browser preferences, and opted-in plugin data) and reload the page. Settings not in the bundle (e.g. from plugins installed after the export) are preserved. Continue?')) {
+        status.textContent = 'Import cancelled';
+        return;
+    }
+    let bundle;
+    try {
+        bundle = JSON.parse(await file.text());
+    } catch (e) {
+        status.textContent = `Import failed: not valid JSON (${e.message})`;
+        return;
+    }
+
+    status.textContent = 'Importing...';
+    let resp, data;
+    try {
+        resp = await fetch('/api/settings/import', {
+            method: 'POST',
+            headers: { 'Content-Type': 'application/json' },
+            body: JSON.stringify(bundle),
+        });
+        data = await resp.json();
+    } catch (e) {
+        status.textContent = `Import failed: ${e.message}`;
+        return;
+    }
+    // Two failure shapes to surface: our own validation handler
+    // returns `{ok: false, error: "..."}`, but if the body fails
+    // FastAPI's request-level validation (e.g. top-level value is
+    // an array, not an object), the response is the framework's
+    // `{detail: ...}` shape with no `ok` key. `resp.ok` distinguishes
+    // both from success without depending on which path produced
+    // the failure.
+    if (!resp.ok || data.ok === false) {
+        let msg = data.error;
+        if (!msg && data.detail) {
+            msg = typeof data.detail === 'string'
+                ? data.detail
+                : JSON.stringify(data.detail);
+        }
+        status.textContent = `Import failed: ${msg || `HTTP ${resp.status}`}`;
+        return;
+    }
+
+    // Server applied successfully. Now apply the localStorage portion as
+    // a MERGE (not clear+restore): keys in the bundle overwrite, keys
+    // present locally but absent from the bundle are preserved. This
+    // matters when a plugin was installed *after* the export — wiping
+    // its localStorage would erase first-run defaults the plugin set on
+    // load, leaving it in a worse state than before the import. The
+    // tradeoff is that orphan keys from removed plugins or renamed key
+    // schemes also linger; cleaning those up is the user's job.
+    const ls = bundle.local_storage;
+    if (ls && typeof ls === 'object') {
+        try {
+            for (const [key, value] of Object.entries(ls)) {
+                if (typeof value === 'string') localStorage.setItem(key, value);
+            }
+        } catch (e) {
+            // Quota exceeded / private mode etc. Server side already
+            // committed, so we surface the partial state rather than
+            // pretending it succeeded.
+            status.textContent = `Server applied, but localStorage write failed: ${e.message}`;
+            return;
+        }
+    }
+
+    const warnings = (data.warnings || []).join('; ');
+    status.textContent = warnings ? `Imported with warnings: ${warnings}. Reloading...` : 'Imported. Reloading...';
+    setTimeout(() => location.reload(), 800);
+}

static/js/transport.js

@@ -0,0 +1,377 @@
+// The playback transport — the play/pause/seek core, and the two clocks it reads.
+//
+// WHY THIS IS A MODULE AND NOT A HOOK BUNDLE. Every carve before this one ADDED host
+// hooks: a module pulled out of app.js still had to call back into it. This one SUBTRACTS
+// them. count-in, juce-audio, loops, and section-practice were all reaching through the
+// seam for the same handful of names — _audioSeek, _audioTime, setPlayButtonState,
+// _songEventPayload, jucePlayer. Those names have an owner, and it isn't app.js. Give
+// them one and the four consumers import them directly:
+//
+//     count-in.js    5 hooks -> 0        juce-audio.js  4 hooks -> 0
+//     loops.js       6 hooks -> 4        section-practice.js  10 hooks -> 7
+//
+// A hook is a cycle you agreed to live with. An import is a dependency you actually have.
+// Prefer the import whenever the name has a real owner.
+//
+// TWO THINGS DELIBERATELY LEFT IN app.js, both for the same reason — they would close a
+// cycle, and app.js is the root, so it can import from both sides for free:
+//
+//   * _currentPlaybackSnapshot  reads loopA/loopB from ./loops.js, and loops.js imports
+//                               this module. The dependency scan MISSED this at first: it
+//                               only walked app.js's own top-level decls, and loopA stopped
+//                               being one the moment loops.js was carved out. Any scan of a
+//                               partly-carved monolith has to resolve the imports too.
+//   * restartCurrentSong        calls _cancelCountIn() from ./count-in.js, which imports
+//                               this module.
+//
+// The seek generation (_audioSeekGen) stays PRIVATE. It has exactly one writer —
+// _resetAudioSeekState(), right here — so readers get audioSeekGen() and nobody outside
+// can desync it. That is strictly better than the host hook it replaces, which handed out
+// a getter and left the writer in app.js.
+import { audio } from './audio-el.js';
+import { S } from './player-state.js';
+
+// Sync the play/pause button's icon and accessible state in one place so
+// screen readers, tooltips, and aria-pressed stay aligned with playback.
+// Updates the existing <img> child's src in place rather than rewriting
+// innerHTML, so any future children (fallback label, loading spinner, …)
+// survive state changes.
+export function setPlayButtonState(isPlaying) {
+    const btn = document.getElementById('btn-play');
+    if (!btn) return;
+    const label = isPlaying ? 'Pause' : 'Play';
+    const icon = isPlaying ? 'pause' : 'play';
+    let img = btn.querySelector('img.button-icon-svg');
+    if (!img) {
+        img = document.createElement('img');
+        img.className = 'button-icon-svg';
+        img.alt = '';
+        img.setAttribute('aria-hidden', 'true');
+        btn.appendChild(img);
+    }
+    img.src = `/static/svg/${icon}.svg`;
+    btn.setAttribute('aria-label', label);
+    btn.setAttribute('aria-pressed', isPlaying ? 'true' : 'false');
+    btn.title = label;
+}
+
+// ── Player ───────────────────────────────────────────────────────────────
+// `audio` now lives in ./js/audio-el.js so carved-out modules can reach the
+// player without importing app.js back (which would close a cycle). Same
+// element, same handle, same lookup — just imported instead of declared here.
+let _lastSongPositionEventAt = 0;
+
+export function _emitSongPositionChanged(time, duration) {
+    const now = Date.now();
+    if (now - _lastSongPositionEventAt < 250) return;
+    _lastSongPositionEventAt = now;
+    const payload = (typeof _songEventPayload === 'function') ? _songEventPayload() : { time };
+    window.feedBack.emit('song:position-changed', Object.assign(payload, { duration }));
+}
+
+export const jucePlayer = {
+    _timer: null,
+    _pos: 0,
+    _dur: 0,
+    _pollAt: 0,    // performance.now() when _pos was last set
+    _polling: false,
+    _speed: 1,
+    get currentTime() {
+        if (!this._polling) return this._pos;
+        // Interpolate between IPC polls so highway motion is smooth at 60fps
+        // Scale by _speed so at 0.7x the interpolated clock advances 0.7s/s
+        const elapsed = (performance.now() - this._pollAt) / 1000;
+        return Math.min(this._pos + elapsed * this._speed, this._dur > 0 ? this._dur : Infinity);
+    },
+    get duration() { return this._dur; },
+    async play() {
+        try {
+            await window.feedBackDesktop.audio.startBacking();
+        } catch (err) {
+            console.warn('[jucePlayer] startBacking failed:', err);
+            return false;
+        }
+        this._startPolling();
+        return true;
+    },
+    async pause() {
+        // Snapshot the interpolated position before stopping the poll so
+        // _pos stays at the visible pause point rather than jumping back
+        // to the last raw IPC sample (which can be up to 100ms behind).
+        this._pos = this.currentTime;
+        this._pollAt = performance.now();
+        this._stopPolling();
+        try {
+            await window.feedBackDesktop.audio.stopBacking();
+        } catch (err) {
+            console.warn('[jucePlayer] stopBacking failed:', err);
+        }
+    },
+    async seek(s) {
+        const prev = this._pos;
+        this._pos = s;
+        this._pollAt = performance.now();
+        try {
+            await window.feedBackDesktop.audio.seekBacking(s);
+        } catch (err) {
+            console.warn('[jucePlayer] seekBacking failed:', err);
+            this._pos = prev;
+            this._pollAt = performance.now();
+        }
+    },
+    _startPolling() {
+        this._stopPolling();
+        this._polling = true;
+        this._pollAt = performance.now();
+        const self = this;
+        function scheduleNext() {
+            self._timer = setTimeout(async () => {
+                if (!self._polling) return;
+                try {
+                    self._pos = await window.feedBackDesktop.audio.getBackingPosition();
+                    self._pollAt = performance.now();
+                    _emitSongPositionChanged(self.currentTime, self.duration || null);
+                } catch (err) {
+                    console.warn('[jucePlayer] position poll failed:', err);
+                } finally {
+                    if (self._polling) scheduleNext();
+                }
+            }, 100);
+        }
+        scheduleNext();
+    },
+    _stopPolling() {
+        this._polling = false;
+        if (this._timer) { clearTimeout(this._timer); this._timer = null; }
+    },
+    setRate(rate) {
+        this._pos = this.currentTime;
+        this._pollAt = performance.now();
+        this._speed = rate;
+    },
+    async stop() {
+        await this.pause();
+        this._pos = 0;
+        this._dur = 0;
+        this._pollAt = 0;
+        this._speed = 1;
+    },
+};
+
+export function _audioTime() { return window._juceMode ? jucePlayer.currentTime : audio.currentTime; }
+
+export function _audioDuration() { return window._juceMode ? jucePlayer.duration : audio.duration; }
+
+// Canonical payload for song:play/song:pause/song:ended. Plugins anchor
+// their own clocks against `perfNow` (a monotonic timestamp at the same
+// moment audio reports `audioT`) so they don't have to chase the chart
+// clock with a follow-up call. `time` is kept as an alias for `audioT`
+// because pre-existing plugins read e.detail.time.
+export function _songEventPayload() {
+    const audioT = _audioTime();
+    return {
+        time: audioT,
+        audioT,
+        chartT: highway.getTime(),
+        perfNow: performance.now(),
+    };
+}
+
+export function _markPlaybackPaused() {
+    S.isPlaying = false;
+    setPlayButtonState(false);
+    if (window.feedBack) {
+        window.feedBack.isPlaying = false;
+        window.feedBack.emit('song:pause', _songEventPayload());
+    }
+}
+
+export function _markPlaybackResumed() {
+    S.isPlaying = true;
+    setPlayButtonState(true);
+    if (window.feedBack) {
+        window.feedBack.isPlaying = true;
+        const payload = _songEventPayload();
+        window.feedBack.emit('song:play', payload);
+        window.feedBack.emit('song:resume', payload);
+    }
+}
+
+export function _emitPlaybackStopped(time, screen = 'playback-command') {
+    if (window.feedBack) window.feedBack.emit('song:stop', { time: time || 0, screen });
+}
+
+export function _waitForSongReady(expectedSeekGen, timeoutMs = 10000) {
+    if (!window.feedBack || typeof window.feedBack.on !== 'function') return Promise.resolve(false);
+    return new Promise(resolve => {
+        let timer = null;
+        const done = value => {
+            if (timer !== null) clearTimeout(timer);
+            window.feedBack.off('song:ready', onReady);
+            resolve(value);
+        };
+        const onReady = () => done(expectedSeekGen == null || expectedSeekGen === _audioSeekGen);
+        window.feedBack.on('song:ready', onReady);
+        timer = setTimeout(() => done(false), timeoutMs);
+    });
+}
+
+// Serializes seeks so concurrent callers (e.g. user ⏪ during a loop wrap)
+// don't interleave their from/to reads — each call captures `from` only
+// once the previous seek + emit have completed. The generation token
+// lets session teardown invalidate queued seeks so they don't run against
+// the new player and emit a stale song:seek.
+let _audioSeekChain = Promise.resolve();
+
+let _audioSeekGen = 0;
+
+export function _resetAudioSeekState() {
+    // Bump the generation — in-flight chain callbacks see the mismatch on
+    // their next guard check and short-circuit (no emit, no further state
+    // mutation by us). Don't reset the chain head: new seeks must still
+    // queue behind the in-flight old seek's IPC so two `jucePlayer.seek()`
+    // calls can't race in the JUCE backing engine. The queue drains
+    // quickly because each subsequent old-gen step bails on the first
+    // guard the moment its predecessor resolves.
+    _audioSeekGen++;
+}
+
+// Time-box the JUCE IPC so a single hung seek can't block the global
+// _audioSeekChain forever (which would freeze every subsequent reposition
+// path: seekBy, loop-wrap, jump-fix, shimmed audio.currentTime).
+const _JUCE_SEEK_TIMEOUT_MS = 2000;
+
+function _juceSeekWithTimeout(s) {
+    let timer;
+    const seekP = jucePlayer.seek(s);
+    const timeoutP = new Promise((_, reject) => {
+        timer = setTimeout(() => reject(new Error('JUCE seek timed out')), _JUCE_SEEK_TIMEOUT_MS);
+    });
+    // Clear the timer once the race settles either way; without this the
+    // pending timeout keeps the event loop alive (and eventually rejects
+    // an unawaited promise) even after a successful seek.
+    return Promise.race([seekP, timeoutP]).finally(() => clearTimeout(timer));
+}
+
+// Resolves to `{ completed, from, to }`:
+//   - completed: true if the seek ran to completion and emitted song:seek;
+//                false if cancelled by a teardown gen bump (or threw).
+//   - from: chart clock just before the seek (NaN on cancel before from-read).
+//   - to:   verified post-seek clock (NaN on cancel/throw).
+// Callers that fire follow-up work after the seek (count-in, arrangement
+// restore, etc.) should check `completed` so they don't act on a torn-down
+// session. Callers that need the actual landed position (because JUCE may
+// clamp or HTML5 may snap to the seekable range) should read `to` rather
+// than re-using the requested `s`.
+export async function _audioSeek(s, reason) {
+    // Single funnel for every audio repositioning. Emits song:seek so
+    // plugins (notedetect detection-suppression during seek transients,
+    // practice-journal segment tracking) can react to any chart-time
+    // jump regardless of which UI path triggered it. `reason` is a
+    // free-form short string ('seek-by', 'loop-wrap', 'loop-set',
+    // 'arrangement-restore', 'jump-fix') so subscribers can filter.
+    const gen = _audioSeekGen;
+    _audioSeekChain = _audioSeekChain.then(async () => {
+        if (gen !== _audioSeekGen) return { completed: false, from: NaN, to: NaN };
+        const from = _audioTime();
+        if (window._juceMode) await _juceSeekWithTimeout(s);
+        else audio.currentTime = s;
+        if (gen !== _audioSeekGen) return { completed: false, from, to: NaN };
+        // Read the verified post-seek position rather than the requested `s`
+        // so plugins observe the actual clock — JUCE may clamp or roll back,
+        // and HTML5 may snap to the nearest seekable range.
+        const to = _audioTime();
+        // Sync the jump-fix tracker so the next 60Hz tick doesn't see a
+        // legitimate far seek (e.g. saved-loop jump > 30s) as a browser
+        // bug and revert it.
+        S.lastAudioTime = to;
+        // Sync the chart clock too so any song:* emit fired right after
+        // _audioSeek resolves (e.g. the auto-resume song:play in
+        // changeArrangement) sees an in-sync chartT via _songEventPayload.
+        // Without this, chartT lags by one 60Hz tick after a seek.
+        if (typeof highway !== 'undefined' && highway && typeof highway.setTime === 'function') {
+            highway.setTime(to);
+        }
+        window.feedBack.emit('song:seek', { from, to, reason: reason || null });
+        return { completed: true, from, to };
+    }).catch((err) => {
+        // Don't let one failed seek poison subsequent ones.
+        console.warn('[_audioSeek]', err);
+        return { completed: false, from: NaN, to: NaN };
+    });
+    return _audioSeekChain;
+}
+
+// Per-attempt counter for HTML5 audio.play() invocations. Bumped on
+// every play branch entry so a slow rejection from attempt N can't
+// clobber the UI of a newer attempt N+1 within the same session.
+let _playAttemptGen = 0;
+
+export async function togglePlay() {
+    if (window._juceMode) {
+        if (S.isPlaying) {
+            await jucePlayer.pause();
+            S.isPlaying = false;
+            setPlayButtonState(false);
+            window.feedBack.isPlaying = false;
+            window.feedBack.emit('song:pause', _songEventPayload());
+        } else {
+            const started = await jucePlayer.play();
+            if (!started) return; // startBacking() failed — IPC error already logged
+            S.isPlaying = true;
+            setPlayButtonState(true);
+            window.feedBack.isPlaying = true;
+            const payload = _songEventPayload();
+            window.feedBack.emit('song:play', payload);
+            window.feedBack.emit('song:resume', payload);
+        }
+        return;
+    }
+    if (S.isPlaying) {
+        audio.pause(); S.isPlaying = false;
+        setPlayButtonState(false);
+    } else {
+        // Flip the UI optimistically before awaiting the play() Promise so
+        // a quick second click during a slow start (buffering, device
+        // wake, etc.) still enters the pause branch above. Two stale-
+        // resolution guards:
+        //   - _audioSeekGen: bumped in showScreen() teardown and
+        //     playSong(), so a rejection from a torn-down session can't
+        //     touch new-session UI. Survives same-URL reloads.
+        //   - _playAttemptGen: bumped on every play branch entry, so
+        //     within a single session a slow rejection from attempt N
+        //     can't clobber a faster attempt N+1 (Play → Pause → Play).
+        const sessionGen = _audioSeekGen;
+        const attempt = ++_playAttemptGen;
+        S.isPlaying = true;
+        setPlayButtonState(true);
+        try {
+            await audio.play();
+        } catch (err) {
+            if (sessionGen !== _audioSeekGen) return;
+            if (attempt !== _playAttemptGen) return;
+            // An engine reroute (HTML5 -> JUCE) deliberately pauses the <audio>
+            // element mid-migration, which rejects this in-flight play() with an
+            // AbortError even though playback continues on the JUCE transport.
+            // The reroute owns isPlaying / the button while it runs (same guard
+            // the <audio> 'play'/'pause' listeners use); resetting here would
+            // leave the button showing Play while the song keeps playing — the
+            // "two clicks to pause on the first song after a fresh load" bug.
+            if (window._juceRerouteInProgress) return;
+            console.error('[app] audio.play() rejected:', err);
+            S.isPlaying = false;
+            setPlayButtonState(false);
+        }
+    }
+}
+
+export async function seekBy(s) {
+    await _audioSeek(Math.max(0, _audioTime() + s), 'seek-by');
+}
+
+/**
+ * Read-only view of the seek generation. Bumped by _resetAudioSeekState() on session
+ * teardown; callers capture it before an await and compare after, so a resolution from a
+ * torn-down session can't touch new-session state.
+ */
+export function audioSeekGen() { return _audioSeekGen; }

static/v3/index.html

@@ -1275,6 +1275,7 @@
          saved 'off'/'full' motion preference on first paint. -->
     <script defer src="/static/v3/venue-mood-fx.js"></script>
     <script defer src="/static/v3/venue-scene-3d.js"></script>
+    <script defer src="/static/v3/venue-crowd.js"></script>
     <script defer src="/static/v3/playlists.js"></script>
     <script defer src="/static/v3/audio-routing.js"></script>
     <script defer src="/static/v3/live-guitar-tone-source.js"></script>

static/v3/venue-crowd.js

@@ -0,0 +1,637 @@
+/*
+ * fee[dB]ack — Venue crowd video layer (career mode PR1).
+ *
+ * Crossfades pre-rendered crowd-state loop videos behind the highway based on
+ * v3:live-performance-state, plus one-shot reaction stingers. Renders through
+ * two video backdrop planes owned by the highway_3d venue background style
+ * (window.h3dVenueBackdropSetVideo / window.h3dVenueBackdropSetMix).
+ *
+ * Inert unless a venue pack manifest is set — by the career plugin via
+ * v3VenueCrowd.setManifest(), or (dev only) a JSON manifest in localStorage
+ * under feedBack-venue-crowd-dev. With no manifest the static bg plate
+ * behaves exactly as before.
+ */
+(function (root) {
+    'use strict';
+
+    // live-performance-hud state → crowd state.
+    const CROWD_OF_PERF = {
+        smoke: 'bored',
+        recovery: 'bored',
+        idle: 'neutral',
+        steady: 'neutral',
+        strong: 'engaged',
+        fire: 'ecstatic',
+    };
+    const CROWD_STATES = ['bored', 'neutral', 'engaged', 'ecstatic'];
+    const CROWD_RANK = { bored: 0, neutral: 1, engaged: 2, ecstatic: 3 };
+
+    const STABLE_MS = 3000;        // target must hold this long before a switch
+    const DWELL_MS = 8000;         // min time between committed switches
+    const FADE_MS = 1200;          // loop crossfade
+    const STINGER_FADE_MS = 400;   // stinger fade-in/out
+    const STINGER_MIN_GAP_MS = 20000;
+    const STREAK_MILESTONES = [25, 50, 100];
+    const CANPLAY_TIMEOUT_MS = 4000;
+    const DEV_FLAG_KEY = 'feedBack-venue-crowd-dev';
+    const SFX_KEY = 'feedBack-venue-crowd-sfx';   // 'on' | 'off' (default off)
+
+    // ---------------------------------------------------------------------
+    // Pure, clock-injected decision logic (unit-tested in
+    // tests/js/venue_crowd.test.js — keep DOM-free).
+    // ---------------------------------------------------------------------
+
+    function crowdStateOfPerf(perfState) {
+        return CROWD_OF_PERF[String(perfState || '').toLowerCase()] || 'neutral';
+    }
+
+    // Hysteresis: a new target must be observed continuously for STABLE_MS,
+    // and at least DWELL_MS must have passed since the last committed switch.
+    function createCrowdMachine() {
+        let current = 'neutral';
+        let candidate = null;
+        let candidateSince = 0;
+        let lastSwitchAt = -Infinity;
+        return {
+            get current() { return current; },
+            reset() {
+                current = "neutral";
+                candidate = null;
+                lastSwitchAt = -Infinity;
+            },
+            // Feed the latest perf state; returns the new crowd state when a
+            // transition commits, else null.
+            update(perfState, nowMs) {
+                const target = crowdStateOfPerf(perfState);
+                if (target === current) {
+                    candidate = null;
+                    return null;
+                }
+                if (target !== candidate) {
+                    candidate = target;
+                    candidateSince = nowMs;
+                    return null;
+                }
+                if (nowMs - candidateSince < STABLE_MS) return null;
+                if (nowMs - lastSwitchAt < DWELL_MS) return null;
+                current = target;
+                candidate = null;
+                lastSwitchAt = nowMs;
+                return current;
+            },
+        };
+    }
+
+    // Cheer when the streak crosses a milestone (rising edge only).
+    function stingerForStreak(prevStreak, streak) {
+        for (const m of STREAK_MILESTONES) {
+            if (prevStreak < m && streak >= m) return 'cheer';
+        }
+        return null;
+    }
+
+    // End-of-song reaction from final accuracy.
+    function stingerForAccuracy(accuracyPct) {
+        const a = Number(accuracyPct);
+        if (!Number.isFinite(a)) return null;
+        if (a >= 90) return 'cheer';
+        if (a >= 75) return 'clap';
+        return null;
+    }
+
+    // ---------------------------------------------------------------------
+    // Video layer controller (browser only).
+    // ---------------------------------------------------------------------
+
+    const machine = createCrowdMachine();
+    let _manifest = null;       // { loops: {state: url}, stingers: {name: url} }
+    let _venueActive = false;
+    let _videos = [null, null];
+    let _activeLayer = 0;       // layer currently showing the loop
+    let _mix = 0;               // 0 → layer0 visible, 1 → layer1 visible
+    let _fadeRaf = 0;
+    let _stopGen = 0;           // bumped by stop(): invalidates ALL in-flight loads
+    let _boundToRenderer = false;
+    let _pendingLoop = null;    // loop switch deferred by an active stinger
+    let _loadingLoop = null;    // loop currently waiting on canplaythrough
+    let _fadingLoop = null;     // loop currently crossfading in (not yet active)
+    let _stingerUntilEnded = false;
+    let _stingerGen = 0;        // identity for ended/timeout handlers
+    let _introActive = false;
+    let _introGen = 0;
+    let _audioEl = null;        // crowd ambience during the intro flyover
+    let _audioFadeTimer = 0;
+    let _lastStingerAt = -Infinity;
+    let _prevStreak = 0;
+    let _lastAccuracyPct = null; // from perf events; stats:recorded carries none
+    let _bound = false;
+
+    function now() { return Date.now(); }
+
+    function h3d(name) {
+        return root && typeof root[name] === 'function' ? root[name] : null;
+    }
+
+    function normalizeManifest(m) {
+        if (!m || typeof m !== 'object' || !m.loops) return null;
+        const base = typeof m.base === 'string' ? m.base : '';
+        const abs = (u) => (typeof u === 'string' && u ? base + u : '');
+        const loops = {};
+        for (const s of CROWD_STATES) loops[s] = abs(m.loops[s]);
+        if (!CROWD_STATES.every((s) => loops[s])) return null;
+        const stingers = {};
+        for (const k of ['clap', 'cheer']) stingers[k] = abs(m.stingers && m.stingers[k]);
+        const intro = {
+            video: abs(m.intro && m.intro.video),
+            audio: abs(m.intro && m.intro.audio),
+        };
+        const sfx = {
+            up: abs(m.sfx && m.sfx.up),
+            down: abs(m.sfx && m.sfx.down),
+        };
+        return { loops, stingers, intro, sfx };
+    }
+
+    function ensureVideos() {
+        if (!_videos[0] && typeof document !== 'undefined') {
+            for (let i = 0; i < 2; i++) {
+                const v = document.createElement('video');
+                // Same autoplay-safe recipe as the highway_3d video bg style:
+                // muted + playsInline bypasses gesture requirements; same-origin
+                // URLs so VideoTexture never taints.
+                v.muted = true;
+                v.playsInline = true;
+                v.preload = 'auto';
+                v.loop = true;
+                v.style.display = 'none';
+                document.body.appendChild(v);
+                _videos[i] = v;
+            }
+        }
+        bindVideosToRenderer();
+    }
+
+    // The highway_3d plugin (and its globals) can register after the venue
+    // pack starts — e.g. Venue selected at page load, renderer ready later.
+    // Idempotent and retried from start() and the perf-event path so a late
+    // renderer still picks the videos up.
+    function bindVideosToRenderer() {
+        if (_boundToRenderer || !_videos[0]) return;
+        const setVideo = h3d('h3dVenueBackdropSetVideo');
+        if (!setVideo) return;
+        setVideo(0, _videos[0]);
+        setVideo(1, _videos[1]);
+        _boundToRenderer = true;
+        setMix(_mix); // re-push mix the renderer missed while unregistered
+    }
+
+    function setMix(v) {
+        _mix = Math.max(0, Math.min(1, v));
+        const fn = h3d('h3dVenueBackdropSetMix');
+        if (fn) fn(_mix);
+    }
+
+    function cancelFade() {
+        if (_fadeRaf && typeof cancelAnimationFrame === 'function') {
+            cancelAnimationFrame(_fadeRaf);
+        }
+        _fadeRaf = 0;
+    }
+
+    function fadeMixTo(target, durationMs, done) {
+        cancelFade();
+        if (typeof requestAnimationFrame !== 'function') {
+            setMix(target);
+            if (done) done();
+            return;
+        }
+        const from = _mix;
+        const t0 = now();
+        const step = () => {
+            const k = Math.min(1, (now() - t0) / durationMs);
+            setMix(from + (target - from) * k);
+            if (k < 1) {
+                _fadeRaf = requestAnimationFrame(step);
+            } else {
+                _fadeRaf = 0;
+                if (done) done();
+            }
+        };
+        _fadeRaf = requestAnimationFrame(step);
+    }
+
+    // Load url into the video, resolve when it can play through (or after a
+    // timeout — a stalled fetch must not wedge the crowd forever). Tokens are
+    // per-element: a later load on the SAME video (a stinger preempting the
+    // idle layer) cancels this one, but loads on the other layer don't.
+    function loadAndPlay(video, url, loop, cb) {
+        const token = (video._fbCrowdToken = (video._fbCrowdToken || 0) + 1);
+        const gen = _stopGen;
+        let settled = false;
+        const settle = (ok) => {
+            if (settled) return;
+            settled = true;
+            // Cleanup must run even for superseded loads or stale listeners
+            // accumulate on the two persistent elements; only the callback
+            // is gated on still being the current load.
+            video.removeEventListener('canplaythrough', onReady);
+            video.removeEventListener('error', onError);
+            if (token !== video._fbCrowdToken || gen !== _stopGen) return;
+            cb(ok);
+        };
+        const onReady = () => settle(true);
+        const onError = () => settle(false);
+        video.addEventListener('canplaythrough', onReady);
+        video.addEventListener('error', onError);
+        video.loop = loop;
+        video.src = url;
+        video.play().catch(() => { /* browser retries on visibility/gesture */ });
+        setTimeout(() => settle(video.readyState >= 3), CANPLAY_TIMEOUT_MS);
+    }
+
+    function idleLayer() { return _activeLayer === 0 ? 1 : 0; }
+
+    // Crossfade the loop for `state` in on the idle layer.
+    function showLoop(state, fadeMs) {
+        if (!_manifest || !_videos[0]) return;
+        const layer = idleLayer();
+        const video = _videos[layer];
+        _loadingLoop = state;
+        loadAndPlay(video, _manifest.loops[state], true, (ok) => {
+            if (_loadingLoop === state) _loadingLoop = null;
+            if (!ok || !_venueActive) return;
+            _fadingLoop = state;
+            fadeMixTo(layer === 1 ? 1 : 0, fadeMs, () => {
+                // Preempted mid-fade (stinger claimed this layer while we
+                // were still ramping): the layer no longer holds this loop —
+                // promoting it would pause the real loop and hand fade-back
+                // the wrong target.
+                if (_fadingLoop !== state) return;
+                _fadingLoop = null;
+                const old = _videos[_activeLayer];
+                _activeLayer = layer;
+                if (old && !old.paused) old.pause();
+            });
+        });
+    }
+
+    function playStinger(name) {
+        if (!_manifest || !_manifest.stingers[name] || !_videos[0]) return;
+        if (_stingerUntilEnded) return;
+        const t = now();
+        if (t - _lastStingerAt < STINGER_MIN_GAP_MS) return;
+        _lastStingerAt = t;
+        _stingerUntilEnded = true;
+        const layer = idleLayer();
+        const video = _videos[layer];
+        // The stinger reuses the idle layer's element, cancelling any loop
+        // load still in flight there — and idleLayer() is still the fading-in
+        // layer while a crossfade runs (_activeLayer flips on completion), so
+        // a mid-fade loop gets overwritten too. Requeue either for when the
+        // stinger ends (the machine already advanced, nothing re-fires it).
+        const interrupted = _loadingLoop || _fadingLoop;
+        if (interrupted) {
+            // Freeze any in-flight crossfade: its ramp would keep pushing the
+            // mix toward this layer while the stinger replaces the src (loop
+            // vanishing / stinger popping in at full opacity).
+            cancelFade();
+            _pendingLoop = interrupted;
+            _loadingLoop = null;
+            _fadingLoop = null;
+        }
+        // A loop switch deferred (or preempted) by this stinger must play
+        // once the stinger is done OR failed — the machine already advanced,
+        // so nothing re-triggers it later.
+        const flushPending = () => {
+            if (!_pendingLoop || !_venueActive) return;
+            const pending = _pendingLoop;
+            _pendingLoop = null;
+            showLoop(pending, FADE_MS);
+        };
+        const myGen = ++_stingerGen;
+        const back = () => {
+            // Always detach: a handler left behind by a stop()/manifest swap
+            // must not fire into a LATER stinger's lifecycle on this reused
+            // element (the gen check below guards that; the boolean alone
+            // would pass once a new stinger is active).
+            video.removeEventListener('ended', back);
+            if (_stingerGen !== myGen || !_stingerUntilEnded) return;
+            _stingerUntilEnded = false;
+            // Fade back to the loop layer (which kept playing underneath).
+            fadeMixTo(_activeLayer === 1 ? 1 : 0, STINGER_FADE_MS);
+            flushPending();
+        };
+        loadAndPlay(video, _manifest.stingers[name], false, (ok) => {
+            if (!ok || !_venueActive) {
+                _stingerUntilEnded = false;
+                flushPending();
+                return;
+            }
+            video.addEventListener('ended', back);
+            fadeMixTo(layer === 1 ? 1 : 0, STINGER_FADE_MS);
+            // Safety: an `ended` that never fires (decode stall) must not
+            // freeze the crowd on a stinger frame.
+            setTimeout(back, 15000);
+        });
+    }
+
+    function ensureAudio() {
+        if (_audioEl || typeof document === 'undefined') return;
+        _audioEl = document.createElement('audio');
+        _audioEl.preload = 'auto';
+        _audioEl.style.display = 'none';
+        document.body.appendChild(_audioEl);
+    }
+
+    function fadeAudioOut(durationMs) {
+        if (!_audioEl || _audioEl.paused) return;
+        if (_audioFadeTimer) return; // already fading
+        const from = _audioEl.volume;
+        const t0 = now();
+        _audioFadeTimer = setInterval(() => {
+            const k = Math.min(1, (now() - t0) / durationMs);
+            _audioEl.volume = from * (1 - k);
+            if (k >= 1) {
+                clearInterval(_audioFadeTimer);
+                _audioFadeTimer = 0;
+                _audioEl.pause();
+            }
+        }, 50);
+    }
+
+    function stopAudio() {
+        if (_audioFadeTimer) { clearInterval(_audioFadeTimer); _audioFadeTimer = 0; }
+        if (_audioEl && !_audioEl.paused) _audioEl.pause();
+    }
+
+    // One-shot flyover intro on song load: video flies from the back of the
+    // room onto the stage, crowd ambience plays and ducks out as the song
+    // starts (song:play) or as the flyover lands, whichever comes first.
+    function playIntro() {
+        if (!_manifest || !_manifest.intro || !_manifest.intro.video || !_videos[0]) {
+            return false;
+        }
+        const myGen = ++_introGen;
+        _introActive = true;
+        const layer = idleLayer();
+        const video = _videos[layer];
+        const land = () => {
+            if (_introGen !== myGen || !_introActive) return;
+            _introActive = false;
+            video.removeEventListener('ended', land);
+            fadeAudioOut(1200);
+            const pending = _pendingLoop;
+            _pendingLoop = null;
+            showLoop(pending || machine.current, 400);
+        };
+        loadAndPlay(video, _manifest.intro.video, false, (ok) => {
+            if (_introGen !== myGen) return;
+            if (!ok || !_venueActive) {
+                // Failed intro must not leave the song loop-less: fall back
+                // to the normal loop exactly like the no-intro path.
+                _introActive = false;
+                if (_venueActive) showLoop(machine.current, FADE_MS);
+                return;
+            }
+            fadeMixTo(layer === 1 ? 1 : 0, 300);
+            video.addEventListener('ended', land);
+            setTimeout(land, 15000); // decode-stall safety
+            if (_manifest.intro.audio) {
+                ensureAudio();
+                _audioEl.src = _manifest.intro.audio;
+                _audioEl.volume = 1;
+                // The user's play gesture precedes song:loaded, so autoplay
+                // with sound is normally allowed; degrade silently if not.
+                _audioEl.play().catch(() => { /* no gesture yet */ });
+                // start ducking shortly before the flyover lands
+                video.addEventListener('timeupdate', function duck() {
+                    if (video.duration && video.duration - video.currentTime < 1.5) {
+                        video.removeEventListener('timeupdate', duck);
+                        fadeAudioOut(1400);
+                    }
+                });
+            }
+        });
+        return true;
+    }
+
+    let _sfxEl = null;
+
+    function sfxEnabled() {
+        try { return localStorage.getItem(SFX_KEY) === 'on'; } catch (_) { return false; }
+    }
+
+    // One-shot crowd reaction on committed mood transitions (toggleable):
+    // up the ladder → cheer, down → boos. Committed transitions are already
+    // hysteresis-limited, so this can't spam.
+    function playMoodSfx(direction) {
+        if (!sfxEnabled() || !_manifest || !_manifest.sfx || _introActive) return;
+        const url = direction > 0 ? _manifest.sfx.up : _manifest.sfx.down;
+        if (!url || typeof document === 'undefined') return;
+        if (!_sfxEl) {
+            _sfxEl = document.createElement('audio');
+            _sfxEl.preload = 'auto';
+            _sfxEl.style.display = 'none';
+            document.body.appendChild(_sfxEl);
+        }
+        _sfxEl.src = url;
+        _sfxEl.volume = 0.6;
+        _sfxEl.play().catch(() => { /* pre-gesture; skip silently */ });
+    }
+
+    function onSongPlay() {
+        // Song audio starting is the hard cue: the ambience must yield.
+        fadeAudioOut(1000);
+    }
+
+    function onPerformanceState(e) {
+        if (!_venueActive || !_manifest) return;
+        bindVideosToRenderer();
+        const d = (e && e.detail) || {};
+        // Number(null) === 0: HUD reset events (accuracyPct: null) must not
+        // wipe the value the end-of-song stinger reads via stats:recorded.
+        if (d.accuracyPct != null && Number.isFinite(Number(d.accuracyPct))) {
+            _lastAccuracyPct = Number(d.accuracyPct);
+        }
+        const streak = Number(d.streak) || 0;
+        const sting = stingerForStreak(_prevStreak, streak);
+        _prevStreak = streak;
+        if (sting && !_introActive && CROWD_RANK[machine.current] >= CROWD_RANK.neutral) {
+            playStinger(sting);
+        }
+        const prevRank = CROWD_RANK[machine.current];
+        const next = machine.update(d.state, now());
+        if (next) {
+            playMoodSfx(CROWD_RANK[next] - prevRank);
+            // A stinger or the intro owns the idle layer; defer the switch.
+            if (_stingerUntilEnded || _introActive) _pendingLoop = next;
+            else showLoop(next, FADE_MS);
+        }
+    }
+
+    function onSongLoaded() {
+        machine.reset();
+        _prevStreak = 0;
+        _lastAccuracyPct = null;
+        // Abort any stinger/pending state from the previous song: its ended
+        // handler must not fade back into the old song's layers.
+        cancelFade();
+        _stingerGen++;
+        _introGen++;
+        _stingerUntilEnded = false;
+        _introActive = false;
+        stopAudio();
+        _pendingLoop = null;
+        _loadingLoop = null;
+        _fadingLoop = null;
+        if (_venueActive && _manifest) {
+            if (!playIntro()) showLoop(machine.current, FADE_MS);
+        }
+    }
+
+    function onStatsRecorded() {
+        if (!_venueActive || !_manifest) return;
+        // stats:recorded carries only {filename, arrangement} — the accuracy
+        // comes from the last v3:live-performance-state of the finished song.
+        const sting = stingerForAccuracy(_lastAccuracyPct);
+        _lastAccuracyPct = null; // one reaction per song
+        if (sting) {
+            _lastStingerAt = -Infinity; // end-of-song reaction always allowed
+            playStinger(sting);
+        }
+    }
+
+    function start() {
+        ensureVideos();
+        if (!_videos[0]) return;
+        _prevStreak = 0;
+        // Boot straight into the current machine state on the active layer.
+        const video = _videos[_activeLayer];
+        loadAndPlay(video, _manifest.loops[machine.current], true, (ok) => {
+            if (!ok || !_venueActive) return;
+            setMix(_activeLayer === 1 ? 1 : 0);
+        });
+    }
+
+    function stop() {
+        cancelFade();
+        _stopGen++;
+        _stingerGen++;
+        _introGen++;
+        _introActive = false;
+        stopAudio();
+        if (_sfxEl && !_sfxEl.paused) _sfxEl.pause();
+        _stingerUntilEnded = false;
+        _pendingLoop = null;
+        _loadingLoop = null;
+        _fadingLoop = null;
+        for (const v of _videos) {
+            if (v && !v.paused) v.pause();
+        }
+        // Unbind from the renderer: a paused video still holds its last
+        // frame, and the venue style keeps a bound plane visible whenever
+        // videoWidth > 0 — without this a removed pack would leave a frozen
+        // crowd frame over the static plate. start() re-binds.
+        const setVideo = h3d('h3dVenueBackdropSetVideo');
+        if (_boundToRenderer && setVideo) {
+            setVideo(0, null);
+            setVideo(1, null);
+        }
+        _boundToRenderer = false;
+        // Mix and active layer must reset together: mix 0 shows layer 0, so a
+        // restart that left _activeLayer at 1 would flash layer 0's stale
+        // frame until the new loop loads.
+        _activeLayer = 0;
+        setMix(0);
+    }
+
+    function setVenueActive(on) {
+        const next = !!on;
+        if (next === _venueActive) {
+            // Re-activation (e.g. viz:renderer:ready after a late plugin
+            // load): don't restart the loop, but do retry renderer binding.
+            if (next && _manifest) bindVideosToRenderer();
+            return;
+        }
+        _venueActive = next;
+        if (_venueActive && _manifest) start();
+        else stop();
+    }
+
+    function setManifest(m) {
+        const norm = normalizeManifest(m);
+        _manifest = norm;
+        if (_venueActive) {
+            // Full stop first even when replacing pack-for-pack: it bumps
+            // _stopGen so an in-flight load from the OLD manifest can't
+            // settle and fade a stale URL in after the new pack starts.
+            stop();
+            if (norm) start();
+        }
+    }
+
+    function readDevManifest() {
+        try {
+            const raw = localStorage.getItem(DEV_FLAG_KEY);
+            if (!raw) return null;
+            return JSON.parse(raw);
+        } catch (_) {
+            return null;
+        }
+    }
+
+    function bindRuntime() {
+        if (_bound) return;
+        _bound = true;
+        const sm = root && root.feedBack;
+        if (sm && typeof sm.on === 'function') {
+            sm.on('v3:live-performance-state', onPerformanceState);
+            sm.on('stats:recorded', onStatsRecorded);
+            // A new song must not inherit the previous song's crowd mood
+            // through the hysteresis/dwell window.
+            sm.on('song:loaded', onSongLoaded);
+            sm.on('song:play', onSongPlay);
+        }
+        const dev = readDevManifest();
+        if (dev && !_manifest) setManifest(dev);
+    }
+
+    function getState() {
+        return {
+            venueActive: _venueActive,
+            hasManifest: !!_manifest,
+            crowdState: machine.current,
+            activeLayer: _activeLayer,
+            mix: _mix,
+            stingerActive: _stingerUntilEnded,
+            introActive: _introActive,
+        };
+    }
+
+    const api = {
+        CROWD_STATES,
+        STABLE_MS,
+        DWELL_MS,
+        crowdStateOfPerf,
+        createCrowdMachine,
+        stingerForStreak,
+        stingerForAccuracy,
+        normalizeManifest,
+        setManifest,
+        setVenueActive,
+        bindRuntime,
+        getState,
+    };
+
+    if (root) root.v3VenueCrowd = api;
+    if (typeof module !== 'undefined' && module.exports) module.exports = api;
+
+    if (typeof document !== 'undefined') {
+        // Same defer/DOMContentLoaded dance as venue-scene-3d.js.
+        if (document.readyState !== 'complete') {
+            document.addEventListener('DOMContentLoaded', bindRuntime);
+        } else {
+            bindRuntime();
+        }
+    }
+}(typeof window !== 'undefined' ? window : (typeof globalThis !== 'undefined' ? globalThis : null)));

static/v3/venue-scene-3d.js

@@ -96,6 +96,7 @@
         if (_active) {
             syncInstrumentPov();
             syncVenueMotion();
+            syncCrowd(true);
             return;
         }
         _active = true;
@@ -105,6 +106,17 @@
         setH3dMood(_lastMood);
         syncInstrumentPov();
         syncVenueMotion();
+        syncCrowd(true);
+    }
+
+    function syncCrowd(on) {
+        // Reactive crowd video layer (career mode) — inert without a pack.
+        try {
+            if (root && root.v3VenueCrowd &&
+                typeof root.v3VenueCrowd.setVenueActive === 'function') {
+                root.v3VenueCrowd.setVenueActive(!!on);
+            }
+        } catch (_) { /* visual-only */ }
     }
 
     function syncVenueMotion() {
@@ -128,6 +140,7 @@
         _assetsLoaded = false;
         _loadFailed = false;
         setH3dActive(false);
+        syncCrowd(false);
         syncPlaceholderVisibility();
     }
 

tests/conftest.py

@@ -1,6 +1,8 @@
 """Shared pytest fixtures for the feedBack test suite."""
 
+import importlib
 import logging
+import sys
 
 import pytest
 import structlog
@@ -76,3 +78,61 @@ def isolate_logging():
         lg.setLevel(original_level)
         lg.propagate = original_propagate
     structlog.reset_defaults()
+
+
+# ── Plugin-loader isolation ─────────────────────────────────────────────────────
+#
+# Lifted verbatim out of tests/test_plugins.py so more than one test module can drive
+# the real plugins.load_plugins(). It has to be ONE fixture, not a copy per file:
+# load_plugins() mutates sys.path, sys.modules, PENDING_PLUGINS and LOADED_PLUGINS, and a
+# partial restore makes the suite order- and environment-dependent (Codex [P2] on
+# test_plugin_context_contract.py — it was right).
+
+# Bare module names that this test module pre-populates into
+# sys.modules to simulate the bare-import path. Saved/restored by
+# the reset_plugin_state fixture so they don't leak to other test
+# files. Codex / Copilot review on PR for feedBack#33.
+_BARE_NAMES_USED = ("util", "extractor")
+
+
+@pytest.fixture()
+def reset_plugin_state(monkeypatch):
+    """Clear loader module-level state and restore on teardown.
+
+    Saves and restores:
+      * `plugins.LOADED_PLUGINS`
+      * any `plugin_*` keys we add to `sys.modules`
+      * the bare names this module simulates (`util`, `extractor`)
+      * `sys.path` — `plugins.load_plugins()` mutates it
+    Also unsets `FEEDBACK_PLUGINS_DIR` for the test's duration
+    (via monkeypatch) so a CI env that pre-sets it can't leak
+    real user plugins into a tmp_path-driven test. Per-module
+    locks are owned by the standard import system
+    (`importlib._bootstrap._module_locks`) and are not our
+    responsibility to reset.
+    """
+    monkeypatch.delenv("FEEDBACK_PLUGINS_DIR", raising=False)
+    plugins = importlib.import_module("plugins")
+    saved_loaded = list(plugins.LOADED_PLUGINS)
+    saved_pending = dict(plugins.PENDING_PLUGINS)
+    saved_modules = {k: v for k, v in sys.modules.items() if k.startswith("plugin_")}
+    saved_bare = {k: sys.modules[k] for k in _BARE_NAMES_USED if k in sys.modules}
+    saved_path = list(sys.path)
+    plugins.LOADED_PLUGINS.clear()
+    plugins.PENDING_PLUGINS.clear()
+    for k in list(sys.modules):
+        if k.startswith("plugin_") or k in _BARE_NAMES_USED:
+            del sys.modules[k]
+    try:
+        yield plugins
+    finally:
+        plugins.LOADED_PLUGINS.clear()
+        plugins.LOADED_PLUGINS.extend(saved_loaded)
+        plugins.PENDING_PLUGINS.clear()
+        plugins.PENDING_PLUGINS.update(saved_pending)
+        for k in list(sys.modules):
+            if k.startswith("plugin_") or k in _BARE_NAMES_USED:
+                del sys.modules[k]
+        sys.modules.update(saved_modules)
+        sys.modules.update(saved_bare)
+        sys.path[:] = saved_path

tests/js/autoplay_exit.test.js

@@ -14,10 +14,16 @@ const vm = require('node:vm');
 const { extractFunction } = require('./test_utils');
 
 const APP_JS = path.join(__dirname, '..', '..', 'static', 'app.js');
+// _autoplayExitEnabled was carved out into static/js/player-controls.js (R3a); the
+// auto-exit machinery around it (_clearAutoExit, holdAutoExit, _resolvePlayerOrigin)
+// stayed in app.js.
+const CONTROLS_JS = path.join(__dirname, '..', '..', 'static', 'js', 'player-controls.js');
 const SRC = fs.readFileSync(APP_JS, 'utf8');
+// the module is ESM; these sandboxes evaluate plain script text
+const CONTROLS_SRC = fs.readFileSync(CONTROLS_JS, 'utf8').replace(/^export /gm, '');
 
 function runEnabled(stored) {
-    const fnSrc = extractFunction(SRC, 'function _autoplayExitEnabled(');
+    const fnSrc = extractFunction(CONTROLS_SRC, 'function _autoplayExitEnabled(');
     const sandbox = {
         localStorage: {
             getItem: () => {

tests/js/host_contract.test.js

@@ -0,0 +1,113 @@
+// The host-seam contract: the hooks the modules USE must be exactly the hooks
+// app.js WIRES.
+//
+// This is the test that makes the seam safe. static/js/host.js already throws at
+// runtime when an unwired hook is read — but a runtime throw only fires if the
+// broken path actually executes, and the entire danger of a host seam is the paths
+// that DON'T run in a smoke test. That is not hypothetical: the plugin loader's
+// seam defaulted a hook to `() => {}`, and a dropped wiring line would have left
+// the viz picker silently not refreshing with no test, boot check, or bot noticing.
+//
+// So this closes it statically. Rename a hook in app.js, drop a line from the
+// configureHost({…}) call, or typo a `host.foo` in a module, and CI fails — on a
+// path nobody ever ran.
+//
+// It is deliberately symmetric:
+//   * used but not wired  -> a latent crash (host.js would throw at runtime)
+//   * wired but not used  -> dead weight, and usually the fossil of a rename
+// Both fail.
+
+const { test } = require('node:test');
+const assert = require('node:assert/strict');
+const fs = require('node:fs');
+const path = require('node:path');
+
+const ROOT = path.join(__dirname, '..', '..');
+const APP_JS = path.join(ROOT, 'static', 'app.js');
+const JS_DIR = path.join(ROOT, 'static', 'js');
+
+// Strip comments, so prose about `host.foo` in a header block is not read as a call
+// site.
+//
+// NOTHING ELSE. An earlier version also tried to strip import statements (to stop
+// `from './host.js'` reading as a hook called `js`) and its `[\s\S]*?` spanned lines
+// and silently ate 14,000 characters of the file — including, in the bite test, the
+// very drift it was supposed to catch. A guard with a hole in it is worse than no
+// guard, because you trust it. The `host.js` path is excluded far more cheaply,
+// below, by refusing a match followed by a quote.
+function scrub(src) {
+    return src
+        .replace(/\/\*[\s\S]*?\*\//g, '')
+        .replace(/^\s*\/\/[^\n]*$/gm, '');
+}
+
+// `host.<name>` — but not `host.js'` from the `from './host.js'` import path, which is
+// the one string in these files that looks like a hook and isn't.
+//
+// The trailing class must forbid a WORD character as well as a quote. With only
+// `(?!['"])`, `host.js'` fails on `js` (a quote follows), then BACKTRACKS to `j` —
+// where the next char is `s`, not a quote — and happily reports a hook called `j`.
+// Forbidding `[\w$]` too leaves it nowhere to backtrack to.
+const HOOK_RE = /(?<![\w$.])host\.([A-Za-z_$][\w$]*)(?![\w$'"])/g;
+
+/** Every `host.<name>` referenced by a carved module. */
+function hooksUsed() {
+    const used = new Map();   // name -> [files]
+    for (const file of fs.readdirSync(JS_DIR)) {
+        if (!file.endsWith('.js') || file === 'host.js') continue;
+        const raw = fs.readFileSync(path.join(JS_DIR, file), 'utf8');
+        if (!/from\s+'\.\/host\.js'/.test(raw)) continue;
+        for (const m of scrub(raw).matchAll(HOOK_RE)) {
+            if (!used.has(m[1])) used.set(m[1], []);
+            used.get(m[1]).push(file);
+        }
+    }
+    return used;
+}
+
+/** Every hook app.js passes to configureHost({ … }). */
+function hooksWired() {
+    const src = scrub(fs.readFileSync(APP_JS, 'utf8'));
+    // NB the closing brace is INDENTED (the call sits inside the boot function), so
+    // anchoring on `\n});` at column 0 runs straight past it and swallows the next
+    // object literal in the file — which is how this first read 77 "hooks", most of
+    // them app.js's window contract.
+    const call = src.match(/configureHost\(\{([\s\S]*?)\n\s*\}\);/);
+    if (!call) return null;   // no seam wired yet — fine until there is one
+    const wired = new Set();
+    for (const m of call[1].matchAll(/(?:^|,)\s*([A-Za-z_$][\w$]*)\s*(?=[,:}]|$)/gm)) {
+        wired.add(m[1]);
+    }
+    return wired;
+}
+
+test('every host.<hook> a module uses is wired by app.js', () => {
+    const used = hooksUsed();
+    if (used.size === 0) return;                      // no consumers yet
+    const wired = hooksWired();
+    assert.ok(wired, 'modules import ./host.js but app.js never calls configureHost({ … })');
+
+    const missing = [...used.keys()]
+        .filter((h) => !wired.has(h))
+        .map((h) => `${h} (used in ${used.get(h).join(', ')})`);
+
+    assert.deepEqual(
+        missing, [],
+        'these hooks are read by a module but never wired by app.js — they would throw at runtime, '
+        + 'on whatever path happens to reach them',
+    );
+});
+
+test('every hook app.js wires is actually used by a module', () => {
+    const wired = hooksWired();
+    if (!wired || wired.size === 0) return;
+    const used = hooksUsed();
+
+    const unused = [...wired].filter((h) => !used.has(h));
+
+    assert.deepEqual(
+        unused, [],
+        'these hooks are wired by app.js but no module reads them — dead weight, and usually '
+        + 'the fossil of a rename that left the other half behind',
+    );
+});

tests/js/juce_engine_reroute.test.js

@@ -1,4 +1,4 @@
-// Behavioral tests for the JUCE engine-reroute watcher in static/app.js.
+// Behavioral tests for the JUCE engine-reroute watcher in static/js/juce-audio.js.
 //
 // The watcher (an IIFE, `_installJuceEngineRoutingWatcher`) migrates a loaded
 // song between the HTML5 <audio> element and the native JUCE backing transport
@@ -14,14 +14,15 @@ const fs = require('node:fs');
 const path = require('node:path');
 const vm = require('node:vm');
 
-const APP_JS = path.join(__dirname, '..', '..', 'static', 'app.js');
+// The JUCE audio shims were carved out of app.js into their own module (R3a).
+const APP_JS = path.join(__dirname, '..', '..', 'static', 'js', 'juce-audio.js');
 
 // Brace-balanced extraction of the watcher IIFE, starting at its `(function`
 // and ending after the matching `})();`.
 function extractWatcherIIFE(src) {
     const marker = '(function _installJuceEngineRoutingWatcher() {';
     const start = src.indexOf(marker);
-    assert.ok(start !== -1, 'watcher IIFE not found in app.js');
+    assert.ok(start !== -1, 'watcher IIFE not found in static/js/juce-audio.js');
     const openBrace = src.indexOf('{', start);
     let depth = 1;
     let i = openBrace + 1;
@@ -84,7 +85,11 @@ function makeSandbox({ isAudioRunning, loadBackingTrack, outputType = 'Windows A
             json: () => Promise.resolve({ path: '/local/song.ogg' }),
         }),
         document: { hidden: false },
-        isPlaying: true,
+        // `isPlaying` moved onto the shared player-state container so a carved module
+        // can WRITE it (an imported binding is read-only). The sliced code now reads and
+        // writes S.isPlaying, so the sandbox provides the same container — the
+        // assertions below are unchanged.
+        S: { isPlaying: true, lastAudioTime: 0 },
         audio,
         jucePlayer,
         __calls: calls,
@@ -96,6 +101,17 @@ function makeSandbox({ isAudioRunning, loadBackingTrack, outputType = 'Windows A
 
     const src = fs.readFileSync(APP_JS, 'utf8');
     const iife = extractWatcherIIFE(src);
+    // The shims reach back into app.js through the host seam (static/js/host.js).
+    // Route it at the SAME stubs this sandbox already had — a fresh `() => {}` would
+    // swallow the calls and the assertions below would pass vacuously.
+    sandbox.host = {
+        jucePlayer: () => sandbox.jucePlayer,
+        playSong: (...a) => (sandbox.playSong ? sandbox.playSong(...a) : undefined),
+        _audioSeek: (...a) => (sandbox._audioSeek ? sandbox._audioSeek(...a) : Promise.resolve({ completed: true })),
+        setPlayButtonState: (...a) => (sandbox.setPlayButtonState ? sandbox.setPlayButtonState(...a) : undefined),
+        _songEventPayload: (...a) => (sandbox._songEventPayload ? sandbox._songEventPayload(...a) : ({})),
+        showScreen: (...a) => (sandbox.showScreen ? sandbox.showScreen(...a) : undefined),
+    };
     vm.createContext(sandbox);
     vm.runInContext(iife, sandbox);
     return sandbox;

tests/js/legacy_shim_hits.test.js

@@ -77,6 +77,11 @@ const PLUGIN_LOADER_JS = path.join(ROOT, 'static', 'js', 'plugin-loader.js');
 // The viz layer was carved out of app.js too (R3a).
 const VIZ_JS = path.join(ROOT, 'static', 'js', 'viz.js');
 const LIBRARY_JS = path.join(ROOT, 'static', 'capabilities', 'library.js');
+// The library itself was carved out of app.js into ./static/js/library.js (R3a). Note the
+// two are DIFFERENT files: LIBRARY_JS above is the capability; this is the UI module.
+// syncLibrarySong deliberately stayed behind in app.js — it reaches showScreen/playSong,
+// and moving it would have dragged the whole playback core into the library module.
+const LIBRARY_MODULE_JS = path.join(ROOT, 'static', 'js', 'library.js');
 
 function source(file) {
     // Normalize CRLF: region() slices fixed CHARACTER windows, so on a
@@ -93,16 +98,20 @@ function region(src, needle, length = 1200) {
 
 test('plugin script hydration exposes the current plugin id for legacy registrations', () => {
     const src = source(PLUGIN_LOADER_JS);
-    const block = region(src, 'script.src = `/api/plugins/${plugin.id}/screen.js');
+    // Anchored on the ASSIGNMENT, not the URL literal: the URL is built in
+    // _pluginScriptUrl() now (#879 — a rollback needs a fresh module URL for the whole
+    // import graph), so the old literal no longer appears at the injection site.
+    const block = region(src, 'script.src = _pluginScriptUrl(');
     assert.match(block, /window\.feedBack\._loadingPluginId\s*=\s*plugin\.id/);
     assert.match(block, /delete\s+window\.feedBack\._loadingPluginId/);
 });
 
 test('library providers route through native library capability', () => {
     const src = source(APP_JS);
+    const libModule = source(LIBRARY_MODULE_JS);
     const librarySrc = source(LIBRARY_JS);
-    const loader = region(src, 'async function loadLibraryProviders', 1800);
-    const selector = region(src, 'async function setLibraryProvider(providerId, options = {})', 1600);
+    const loader = region(libModule, 'async function loadLibraryProviders', 1800);
+    const selector = region(libModule, 'async function setLibraryProvider(providerId, options = {})', 1600);
     const sync = region(src, 'async function syncLibrarySong(providerId, songId', 1600);
 
     assert.match(librarySrc, /capabilities\.registerOwner\(['"]library['"]/);

tests/js/loop_api.test.js

@@ -11,11 +11,16 @@ const fs = require('node:fs');
 const path = require('node:path');
 const vm = require('node:vm');
 
+// The A-B loop was carved out of app.js into its own module (R3a). The
+// window.feedBack API surface it is published through stayed in app.js.
+const LOOPS_JS = path.join(__dirname, '..', '..', 'static', 'js', 'loops.js');
 const APP_JS = path.join(__dirname, '..', '..', 'static', 'app.js');
 
-function extractFunction(src, signature) {
+function extractFunction(rawSrc, signature) {
+    // loops.js is an ES module; the vm sandbox evaluates plain script text.
+    const src = rawSrc.replace(/^export /gm, '');
     const start = src.indexOf(signature);
-    if (start === -1) throw new Error(`extractFunction: '${signature}' not found in app.js`);
+    if (start === -1) throw new Error(`extractFunction: '${signature}' not found in static/js/loops.js`);
     let scan = start + signature.length;
     if (src[scan] === '(') {
         let parenDepth = 1;
@@ -44,10 +49,18 @@ function buildSandbox() {
     const seekCalls = [];
     const sectionPracticeModeCalls = [];
     const transportEvents = [];
+    // clearLoop() used to zero section-practice's three selection scalars by hand.
+    // They now live in static/js/section-practice.js, which owns them, so clearLoop
+    // calls its exported resetSelection() instead. This is a SPY, not a stub — the
+    // test below still asserts the reset happens, it just asserts it through the
+    // seam rather than by reaching into someone else's state.
+    const resetSelectionCalls = [];
     const sandbox = {
         seekCalls,
         sectionPracticeModeCalls,
         transportEvents,
+        resetSelectionCalls,
+        resetSelection: () => resetSelectionCalls.push(true),
         // Mutable state (declared as `var` in eval prelude so it lives on
         // the sandbox global and the extracted functions can read/write).
         // The actual values are set below.
@@ -81,6 +94,7 @@ function buildSandbox() {
         // updateLoopUI references formatTime for the label; we don't
         // assert on the label text in these tests, so a stub is enough.
         formatTime: (s) => String(s),
+        _updateEditRegionBtn: () => {},
         window: {
             feedBack: {
                 playback: {
@@ -89,6 +103,19 @@ function buildSandbox() {
             },
         },
     };
+    // The loop module reaches back into app.js through the host seam
+    // (static/js/host.js), so the extracted bodies call host._audioSeek(),
+    // host._audioTime(), and so on. Point the seam at the SAME spies the sandbox
+    // already had: the assertions below are unchanged, they just travel through the
+    // indirection the real code now uses.
+    sandbox.host = {
+        _audioSeek: (...a) => sandbox._audioSeek(...a),
+        _audioTime: () => sandbox._audioTime(),
+        formatTime: (...a) => sandbox.formatTime(...a),
+        _updateEditRegionBtn: () => sandbox._updateEditRegionBtn(),
+        currentFilename: () => 'test-song.sloppak',
+        startCountIn: () => {},
+    };
     vm.createContext(sandbox);
     return sandbox;
 }
@@ -121,7 +148,7 @@ function loadFunctions(sandbox, src) {
 }
 
 test('setLoop mutates loopA/loopB and seeks to A', async () => {
-    const src = fs.readFileSync(APP_JS, 'utf8');
+    const src = fs.readFileSync(LOOPS_JS, 'utf8');
     const sandbox = buildSandbox();
     loadFunctions(sandbox, src);
 
@@ -137,7 +164,7 @@ test('setLoop mutates loopA/loopB and seeks to A', async () => {
 test('setLoop returns false and leaves loopA/loopB untouched on cancelled seek', async () => {
     // Plugin-facing contract: cancelled seek (teardown gen bump) returns
     // false; the loop is NOT armed.
-    const src = fs.readFileSync(APP_JS, 'utf8');
+    const src = fs.readFileSync(LOOPS_JS, 'utf8');
     const sandbox = buildSandbox();
     sandbox._audioSeek = () => Promise.resolve({ completed: false, from: NaN, to: NaN });
     loadFunctions(sandbox, src);
@@ -154,7 +181,7 @@ test('setLoop returns false and leaves loopA/loopB untouched on cancelled seek',
 test('setLoop returns false and leaves loopA/loopB untouched on off-target landing', async () => {
     // JUCE rollback / HTML5 clamp: completed:true but to drifts > 50ms
     // from the requested a. The loop is NOT armed.
-    const src = fs.readFileSync(APP_JS, 'utf8');
+    const src = fs.readFileSync(LOOPS_JS, 'utf8');
     const sandbox = buildSandbox();
     sandbox._audioSeek = (s) => Promise.resolve({ completed: true, from: 0, to: s + 0.5 });
     loadFunctions(sandbox, src);
@@ -172,7 +199,7 @@ test('setLoop coerces string inputs (parseFloat-style)', async () => {
     // loadSavedLoop passes parseFloat(dataset.start) — but the dataset
     // values may already be strings. Number() coercion in setLoop must
     // accept finite numeric strings.
-    const src = fs.readFileSync(APP_JS, 'utf8');
+    const src = fs.readFileSync(LOOPS_JS, 'utf8');
     const sandbox = buildSandbox();
     loadFunctions(sandbox, src);
 
@@ -183,7 +210,7 @@ test('setLoop coerces string inputs (parseFloat-style)', async () => {
 });
 
 test('setLoop rejects non-finite inputs', async () => {
-    const src = fs.readFileSync(APP_JS, 'utf8');
+    const src = fs.readFileSync(LOOPS_JS, 'utf8');
     const sandbox = buildSandbox();
     loadFunctions(sandbox, src);
 
@@ -193,7 +220,7 @@ test('setLoop rejects non-finite inputs', async () => {
 });
 
 test('setLoop rejects b <= a', async () => {
-    const src = fs.readFileSync(APP_JS, 'utf8');
+    const src = fs.readFileSync(LOOPS_JS, 'utf8');
     const sandbox = buildSandbox();
     loadFunctions(sandbox, src);
 
@@ -201,8 +228,8 @@ test('setLoop rejects b <= a', async () => {
     await assert.rejects(() => sandbox.__setLoop(10, 5), /b > a/);
 });
 
-test('clearLoop resets loopA/loopB to null', async () => {
-    const src = fs.readFileSync(APP_JS, 'utf8');
+test('clearLoop resets loopA/loopB to null (and asks section-practice to drop its selection)', async () => {
+    const src = fs.readFileSync(LOOPS_JS, 'utf8');
     const sandbox = buildSandbox();
     loadFunctions(sandbox, src);
 
@@ -211,6 +238,11 @@ test('clearLoop resets loopA/loopB to null', async () => {
     const { loopA, loopB } = sandbox.__getLoop();
     assert.equal(loopA, null);
     assert.equal(loopB, null);
+    assert.equal(
+        sandbox.resetSelectionCalls.length, 1,
+        'clearLoop must ask section-practice to drop its selection (it used to zero the '
+        + 'scalars by hand; the module owns them now)',
+    );
     assert.equal(sandbox.sectionPracticeModeCalls.length, 1);
     assert.equal(sandbox.sectionPracticeModeCalls[0].on, false);
     // Field-wise: vm-context objects break deepStrictEqual across realms.
@@ -218,7 +250,7 @@ test('clearLoop resets loopA/loopB to null', async () => {
 });
 
 test('loop helpers emit transport snapshots by default and can suppress adapter echoes', async () => {
-    const src = fs.readFileSync(APP_JS, 'utf8');
+    const src = fs.readFileSync(LOOPS_JS, 'utf8');
     const sandbox = buildSandbox();
     loadFunctions(sandbox, src);
 
@@ -256,7 +288,7 @@ test('loadSavedLoop funnels through setLoop (no duplicated UI mutation)', () =>
     // re-implementing the loopA/loopB assignment. Catches a future drift
     // where someone "fixes" loadSavedLoop and forgets to keep setLoop in
     // sync.
-    const src = fs.readFileSync(APP_JS, 'utf8');
+    const src = fs.readFileSync(LOOPS_JS, 'utf8');
     const fn = extractFunction(src, 'async function loadSavedLoop(');
     assert.match(fn, /await\s+setLoop\(/, 'loadSavedLoop must call setLoop');
     // The pre-refactor body assigned loopA = parseFloat(...) directly;

tests/js/loop_restart.test.js

@@ -14,7 +14,8 @@ const fs = require('node:fs');
 const path = require('node:path');
 const vm = require('node:vm');
 
-const APP_JS = path.join(__dirname, '..', '..', 'static', 'app.js');
+// startCountIn was carved out of app.js into its own module (R3a).
+const APP_JS = path.join(__dirname, '..', '..', 'static', 'js', 'count-in.js');
 
 // Pull a function body by declaration prefix (e.g. `async function startCountIn`)
 // and brace-matching to the closing brace. Skips an optional `( ... )` param
@@ -55,8 +56,10 @@ function buildSandbox() {
         loopA: 10,
         loopB: 20,
         _countingIn: false,
-        isPlaying: false,
-        lastAudioTime: 0,
+        // isPlaying / lastAudioTime moved onto the shared player-state container
+        // (static/js/player-state.js) so a carved module can WRITE them — an imported
+        // binding is read-only. Same values, same assertions, one indirection.
+        S: { isPlaying: false, lastAudioTime: 0 },
 
         // Browser-ish globals.
         performance: { now: () => Date.now() },
@@ -109,12 +112,23 @@ function buildSandbox() {
         __emitCalls: emitCalls,
         queueMicrotask,
     };
+    // startCountIn was carved into static/js/count-in.js and now reaches back into
+    // app.js through the host seam (static/js/host.js). Point the seam at the SAME
+    // stubs the sandbox already had: the assertions below are unchanged, they just
+    // travel through the indirection the real code now uses.
+    sandbox.host = {
+        _audioSeek: (...a) => sandbox._audioSeek(...a),
+        setPlayButtonState: () => {},
+        _songEventPayload: () => ({}),
+        togglePlay: () => {},
+        jucePlayer: () => sandbox.jucePlayer,
+    };
     vm.createContext(sandbox);
     return sandbox;
 }
 
 test('loop:restart fires once when wrap path runs', async () => {
-    const src = fs.readFileSync(APP_JS, 'utf8');
+    const src = fs.readFileSync(APP_JS, 'utf8').replace(/^export /gm, '');
     const startCountInSrc = extractFunction(src, 'async function startCountIn');
 
     // Sanity check: the change under test is present at all. Catches
@@ -135,8 +149,7 @@ test('loop:restart fires once when wrap path runs', async () => {
         var _countInGen = 0;
         var _countInTimer = null;
         var _countInRaf = 0;
-        var isPlaying = false;
-        var lastAudioTime = 0;
+        var S = { isPlaying: false, lastAudioTime: 0 };
         ${startCountInSrc}
         globalThis.__startCountIn = startCountIn;
     `;
@@ -166,7 +179,7 @@ test('loop:restart aborts when seek lands far from loopA (JUCE rollback)', async
     // _audioSeek resolves with completed:true but r.to !== loopA. The
     // wrap handler must abort instead of running beginCount on the wrong
     // position and emitting a misleading loop:restart.
-    const src = fs.readFileSync(APP_JS, 'utf8');
+    const src = fs.readFileSync(APP_JS, 'utf8').replace(/^export /gm, '');
     const startCountInSrc = extractFunction(src, 'async function startCountIn');
 
     const sandbox = buildSandbox();
@@ -180,8 +193,7 @@ test('loop:restart aborts when seek lands far from loopA (JUCE rollback)', async
         var _countInGen = 0;
         var _countInTimer = null;
         var _countInRaf = 0;
-        var isPlaying = false;
-        var lastAudioTime = 0;
+        var S = { isPlaying: false, lastAudioTime: 0 };
         ${startCountInSrc}
         globalThis.__startCountIn = startCountIn;
         globalThis.__getCountingIn = () => _countingIn;
@@ -202,7 +214,7 @@ test('count-in cancellation token bails delayed callbacks (rewindStep + tick)',
     // teardown can interrupt an in-flight count-in. Behavioral simulation
     // of timer cancellation is out of scope for the static extractor; this
     // verifies the contract is wired into the source.
-    const src = fs.readFileSync(APP_JS, 'utf8');
+    const src = fs.readFileSync(APP_JS, 'utf8').replace(/^export /gm, '');
     const fn = extractFunction(src, 'async function startCountIn');
     // Captures gen at entry
     assert.match(fn, /const gen = _countInGen/, 'startCountIn must capture _countInGen at entry');
@@ -218,7 +230,7 @@ test('loop:restart fires after highway.setTime, before beginCount', () => {
     // Source-order assertion on the A-B wrap path only. Section-practice
     // `opts.immediate` also emits loop:restart but is a separate entry path;
     // the wrap handler lives inside the `_audioSeek(loopA, 'loop-wrap')` then.
-    const src = fs.readFileSync(APP_JS, 'utf8');
+    const src = fs.readFileSync(APP_JS, 'utf8').replace(/^export /gm, '');
     const fn = extractFunction(src, 'async function startCountIn');
     const wrapMarker = "_audioSeek(loopA, 'loop-wrap')";
     const wrapStart = fn.indexOf(wrapMarker);

tests/js/play_button_reroute_guard.test.js

@@ -18,7 +18,7 @@ const vm = require('node:vm');
 
 const { extractFunction } = require('./test_utils');
 
-const APP_JS = path.join(__dirname, '..', '..', 'static', 'app.js');
+const APP_JS = path.join(__dirname, '..', '..', 'static', 'js', 'transport.js');
 const SRC = fs.readFileSync(APP_JS, 'utf8');
 const TOGGLE_PLAY_SRC = extractFunction(SRC, 'async function togglePlay(');
 
@@ -29,8 +29,11 @@ async function runTogglePlayRejecting({ rerouteInProgress }) {
     const buttonStates = [];
     const sandbox = {
         console: { log() {}, warn() {}, error() {} },
-        // not-playing -> togglePlay takes the HTML5 play branch
-        isPlaying: false,
+        // not-playing -> togglePlay takes the HTML5 play branch.
+        // isPlaying / lastAudioTime moved onto the shared player-state container
+        // (static/js/player-state.js) so a carved module can WRITE them — an imported
+        // binding is read-only. Same values, same assertions, one indirection.
+        S: { isPlaying: false, lastAudioTime: 0 },
         _audioSeekGen: 0,
         _playAttemptGen: 0,
         setPlayButtonState(v) { buttonStates.push(v); },
@@ -51,7 +54,7 @@ async function runTogglePlayRejecting({ rerouteInProgress }) {
     vm.createContext(sandbox);
     vm.runInContext(TOGGLE_PLAY_SRC, sandbox, { filename: 'app.js#togglePlay' });
     await vm.runInContext('togglePlay()', sandbox);
-    return { buttonStates, isPlaying: sandbox.isPlaying };
+    return { buttonStates, isPlaying: sandbox.S.isPlaying };
 }
 
 test('reroute-aborted play() leaves the button on Pause (isPlaying stays true)', async () => {

tests/js/playback_app_adapter.test.js

@@ -5,6 +5,10 @@ const path = require('node:path');
 const vm = require('node:vm');
 
 const APP_JS = path.join(__dirname, '..', '..', 'static', 'app.js');
+// SPLIT. _installPlaybackTransportAdapter stayed in app.js — it reads loopA/loopB from
+// ./js/loops.js, and loops.js imports transport, so moving it would close a cycle.
+// _waitForSongReady went with the rest of the seek machinery.
+const TRANSPORT_JS = path.join(__dirname, '..', '..', 'static', 'js', 'transport.js');
 
 function extractFunction(src, signature) {
     const start = src.indexOf(signature);
@@ -54,7 +58,7 @@ function loadReadyHelper(sandbox, src) {
 }
 
 test('_waitForSongReady rejects a ready event from a different audio generation', async () => {
-    const src = fs.readFileSync(APP_JS, 'utf8');
+    const src = fs.readFileSync(TRANSPORT_JS, 'utf8');
     const sandbox = buildReadySandbox();
     loadReadyHelper(sandbox, src);
 
@@ -72,7 +76,7 @@ test('playback adapter scopes startTime readiness and validates seek targets', (
     const src = fs.readFileSync(APP_JS, 'utf8');
     const fn = extractFunction(src, 'function _installPlaybackTransportAdapter()');
 
-    assert.match(fn, /const expectedSeekGen\s*=\s*_audioSeekGen\s*\+\s*1;/);
+    assert.match(fn, /const expectedSeekGen\s*=\s*audioSeekGen\(\)\s*\+\s*1;/);
     assert.match(fn, /_waitForSongReady\(expectedSeekGen\)/);
     assert.match(fn, /const seconds\s*=\s*Number\(time\);/);
     assert.match(fn, /!Number\.isFinite\(seconds\)\s*\|\|\s*seconds\s*<\s*0/);
@@ -84,5 +88,8 @@ test('playback adapter suppresses duplicate HTML5 pause events before emitting c
     const src = fs.readFileSync(APP_JS, 'utf8');
     const fn = extractFunction(src, 'function _installPlaybackTransportAdapter()');
 
-    assert.match(fn, /if \(!window\._juceMode && wasPlaying\) \{\s*isPlaying = false;\s*window\.feedBack\.isPlaying = false;\s*audio\.pause\(\);\s*_markPlaybackPaused\(\);\s*\}/);
+    // isPlaying moved onto the shared player-state container so a carved module can
+    // WRITE it (an imported binding is read-only). window.feedBack.isPlaying — the
+    // public mirror — is unchanged.
+    assert.match(fn, /if \(!window\._juceMode && wasPlaying\) \{\s*S\.isPlaying = false;\s*window\.feedBack\.isPlaying = false;\s*audio\.pause\(\);\s*_markPlaybackPaused\(\);\s*\}/);
 });

tests/js/plugin_loader_script_type.test.js

@@ -19,13 +19,19 @@ const path = require('node:path');
 const PLUGIN_LOADER_JS = path.join(__dirname, '..', '..', 'static', 'js', 'plugin-loader.js');
 const src = fs.readFileSync(PLUGIN_LOADER_JS, 'utf8');
 
-// Isolate the screen.js <script> injection block: from where its src is built
-// to where the element is appended.
+// Isolate the screen.js <script> injection block: from where its src is assigned to
+// where the element is appended.
+//
+// Anchored on the ASSIGNMENT, not on the URL literal. The URL is built in
+// _pluginScriptUrl() now (#879 — a rollback needs a fresh module URL), so the literal
+// '/api/plugins/${plugin.id}/screen.js' appears FURTHER DOWN the file than the block
+// that uses it, and slicing from it ran off the end of the injection block entirely.
+const SRC_ASSIGN = 'script.src = _pluginScriptUrl(';
 function injectionBlock() {
-    const start = src.indexOf('/api/plugins/${plugin.id}/screen.js');
-    assert.ok(start !== -1, 'screen.js injection src not found — loader moved?');
+    const start = src.indexOf(SRC_ASSIGN);
+    assert.ok(start !== -1, 'screen.js src assignment not found — loader moved?');
     const end = src.indexOf('document.body.appendChild(script)', start);
-    assert.ok(end !== -1, 'appendChild(script) not found after screen.js src');
+    assert.ok(end !== -1, 'appendChild(script) not found after the src assignment');
     return src.slice(start, end);
 }
 
@@ -52,7 +58,7 @@ test('the module type is gated, never set unconditionally', () => {
 
 test('the module guard sits before appendChild, after the src assignment', () => {
     const guardAt = src.indexOf('script.type = \'module\'');
-    const srcAt = src.indexOf('/api/plugins/${plugin.id}/screen.js');
+    const srcAt = src.indexOf(SRC_ASSIGN);
     const appendAt = src.indexOf('document.body.appendChild(script)', srcAt);
     assert.ok(guardAt > srcAt && guardAt < appendAt,
         'the module guard must live inside the screen.js injection block');

tests/js/plugin_module_rollback.test.js

@@ -0,0 +1,83 @@
+// #879 — a plugin ROLLBACK must actually re-evaluate a module plugin.
+//
+// ES modules are evaluated once per URL per document. Re-inserting a
+// <script type="module"> whose src the module map has already seen fires `load` but
+// does NOT re-run the body — so rolling back to a version already evaluated this
+// session left the OLD module live while the loader recorded success.
+//
+// The fix puts a generation token in the PATH (/api/plugins/x/g/7/screen.js), not the
+// query, because a relative specifier resolves against the base URL with the query
+// DROPPED — so './src/main.js' would otherwise keep resolving to the same cached URL
+// and the plugin's actual code would never re-run.
+
+const { test } = require('node:test');
+const assert = require('node:assert/strict');
+const fs = require('node:fs');
+const path = require('node:path');
+const vm = require('node:vm');
+
+const { extractFunction } = require('./test_utils');
+const LOADER = path.join(__dirname, '..', '..', 'static', 'js', 'plugin-loader.js');
+
+function makeUrlBuilder() {
+    const src = fs.readFileSync(LOADER, 'utf8');
+    const sandbox = { _evaluatedModules: new Set(), _moduleReloadSeq: 0 };
+    vm.createContext(sandbox);
+    vm.runInContext(`
+        ${extractFunction(src, 'function _pluginScriptUrl(')}
+        globalThis.url = _pluginScriptUrl;
+    `, sandbox);
+    return sandbox.url;
+}
+
+const MOD = { id: 'editor', script_type: 'module' };
+const CLASSIC = { id: 'legacy', script_type: 'classic' };
+
+test('a module plugin first load uses the stable ?v= URL (ETag/304 stays intact)', () => {
+    const url = makeUrlBuilder();
+    assert.equal(url(MOD, '1.0.0', '?v=1.0.0'), '/api/plugins/editor/screen.js?v=1.0.0');
+});
+
+// An UPGRADE has to bust the graph too, and this is the part #879 got wrong. It says
+// "upgrades are fine — a new version yields a new URL". True of screen.js; FALSE of the
+// plugin. Driving a real browser through install -> upgrade -> rollback and counting
+// evaluations of src/main.js gives ONE: the upgrade re-runs the one-line screen.js shim
+// at its new ?v= URL, the shim imports './src/main.js', that resolves to the SAME url,
+// and the module map hands back the already-evaluated old module. So the key here is the
+// plugin ID, not id@version — every re-load of a module plugin needs a fresh path.
+test('an UPGRADE also gets a fresh /g/<n>/ path — a new ?v= does NOT reach the graph', () => {
+    const url = makeUrlBuilder();
+    url(MOD, '1.0.0', '?v=1.0.0');
+    assert.equal(url(MOD, '1.1.0', '?v=1.1.0'), '/api/plugins/editor/g/1/screen.js?v=1.1.0');
+});
+
+test('a ROLLBACK to an already-evaluated version gets a fresh /g/<n>/ PATH', () => {
+    const url = makeUrlBuilder();
+    url(MOD, '1.0.0', '?v=1.0.0');                        // installed
+    url(MOD, '1.1.0', '?v=1.1.0');                        // upgraded   -> /g/1/
+    const back = url(MOD, '1.0.0', '?v=1.0.0');           // rolled back -> /g/2/
+    assert.equal(back, '/api/plugins/editor/g/2/screen.js?v=1.0.0');
+
+    // The token must be in the PATH so a relative import INHERITS it — the whole point.
+    // A query token is dropped by URL resolution and never reaches src/main.js.
+    const resolved = new URL('./src/main.js', `http://h${back}`).pathname;
+    assert.equal(resolved, '/api/plugins/editor/g/2/src/main.js',
+        'the token must reach the module GRAPH, not just the entry point');
+});
+
+test('every re-load gets a distinct URL (no reuse across a bounce)', () => {
+    const url = makeUrlBuilder();
+    url(MOD, '1.0.0', '?v=1.0.0');
+    const seen = new Set();
+    for (const v of ['1.1.0', '1.0.0', '1.1.0', '1.0.0']) seen.add(url(MOD, v, `?v=${v}`));
+    assert.equal(seen.size, 4, 'each re-load must be a URL the module map has never seen');
+});
+
+test('classic-script plugins are untouched — they always re-run on re-insert', () => {
+    const url = makeUrlBuilder();
+    const first = url(CLASSIC, '1.0.0', '?v=1.0.0');
+    url(CLASSIC, '1.1.0', '?v=1.1.0');
+    const back = url(CLASSIC, '1.0.0', '?v=1.0.0');
+    assert.equal(first, '/api/plugins/legacy/screen.js?v=1.0.0');
+    assert.equal(back, first, 'a classic script needs no cache-busting and must not get a /g/ path');
+});

tests/js/renderer_bus_feeder.test.js

@@ -1,4 +1,4 @@
-// Behavioral tests for the renderer-audio bus feeder in static/app.js.
+// Behavioral tests for the renderer-audio bus feeder in static/js/juce-audio.js.
 //
 // The feeder (an IIFE, `_installRendererBusFeeder`) captures renderer-side
 // song audio (stems-plugin WebAudio master, or the core <audio> element) and
@@ -16,12 +16,13 @@ const fs = require('node:fs');
 const path = require('node:path');
 const vm = require('node:vm');
 
-const APP_JS = path.join(__dirname, '..', '..', 'static', 'app.js');
+// The JUCE audio shims were carved out of app.js into their own module (R3a).
+const APP_JS = path.join(__dirname, '..', '..', 'static', 'js', 'juce-audio.js');
 
 function extractFeederIIFE(src) {
     const marker = '(function _installRendererBusFeeder() {';
     const start = src.indexOf(marker);
-    assert.ok(start !== -1, 'feeder IIFE not found in app.js');
+    assert.ok(start !== -1, 'feeder IIFE not found in static/js/juce-audio.js');
     const openBrace = src.indexOf('{', start);
     let depth = 1;
     let i = openBrace + 1;
@@ -123,6 +124,17 @@ function makeSandbox({ isAudioRunning = () => true, exclusive = () => true, disp
     sandbox.globalThis = sandbox;
 
     const src = fs.readFileSync(APP_JS, 'utf8');
+    // The shims reach back into app.js through the host seam (static/js/host.js).
+    // Route it at the SAME stubs this sandbox already had — a fresh `() => {}` would
+    // swallow the calls and the assertions below would pass vacuously.
+    sandbox.host = {
+        jucePlayer: () => sandbox.jucePlayer,
+        playSong: (...a) => (sandbox.playSong ? sandbox.playSong(...a) : undefined),
+        _audioSeek: (...a) => (sandbox._audioSeek ? sandbox._audioSeek(...a) : Promise.resolve({ completed: true })),
+        setPlayButtonState: (...a) => (sandbox.setPlayButtonState ? sandbox.setPlayButtonState(...a) : undefined),
+        _songEventPayload: (...a) => (sandbox._songEventPayload ? sandbox._songEventPayload(...a) : ({})),
+        showScreen: (...a) => (sandbox.showScreen ? sandbox.showScreen(...a) : undefined),
+    };
     vm.createContext(sandbox);
     vm.runInContext(extractFeederIIFE(src), sandbox);
     assert.equal(typeof sandbox.window._reevaluateRendererBus, 'function',

tests/js/section_practice_dismiss.test.js

@@ -15,9 +15,10 @@ const assert = require('node:assert/strict');
 const fs = require('node:fs');
 const path = require('node:path');
 
-const src = fs.readFileSync(path.join(__dirname, '..', '..', 'static', 'app.js'), 'utf8');
+// _installSectionPracticeDismiss was carved out of app.js into its own module (R3a).
+const src = fs.readFileSync(path.join(__dirname, '..', '..', 'static', 'js', 'section-practice.js'), 'utf8');
 const m = src.match(/function _installSectionPracticeDismiss\s*\(\)\s*\{[\s\S]*?\n\}/);
-assert.ok(m, '_installSectionPracticeDismiss() not found in static/app.js');
+assert.ok(m, '_installSectionPracticeDismiss() not found in static/js/section-practice.js');
 const body = m[0];
 
 test('the outside-click dismiss binds in the CAPTURE phase', () => {

tests/js/song_credits_overlay.test.js

@@ -14,8 +14,9 @@ const vm = require('node:vm');
 
 const { extractFunction } = require('./test_utils');
 
-const APP_JS = path.join(__dirname, '..', '..', 'static', 'app.js');
-const SRC = fs.readFileSync(APP_JS, 'utf8');
+// the song-credits overlay was carved out of app.js into its own module (R3a).
+const APP_JS = path.join(__dirname, '..', '..', 'static', 'js', 'count-in.js');
+const SRC = fs.readFileSync(APP_JS, 'utf8').replace(/^export /gm, '');
 
 // Minimal fake DOM element: records className, children, and textContent.
 // Setting textContent clears children (matching real DOM) so we can assert

tests/js/song_event_payload.test.js

@@ -12,7 +12,7 @@ const fs = require('node:fs');
 const path = require('node:path');
 const vm = require('node:vm');
 
-const APP_JS = path.join(__dirname, '..', '..', 'static', 'app.js');
+const APP_JS = path.join(__dirname, '..', '..', 'static', 'js', 'transport.js');
 
 function extractFunction(src, signature) {
     const start = src.indexOf(signature);
@@ -129,11 +129,24 @@ test('every song:play/pause/ended emit uses _songEventPayload', () => {
     );
 });
 
+// CENSUS over the WHOLE frontend, not one file. This test counts call/emit sites, and the
+// carve keeps moving them between app.js and static/js/*.js — point it at a single file
+// and the count silently shrinks as code leaves, which reads as "someone deleted an emit"
+// (or, worse, passes while genuinely missing sites). Read every source that can hold one.
+function allFrontendSources() {
+    const jsDir = path.join(__dirname, '..', '..', 'static', 'js');
+    const parts = [fs.readFileSync(path.join(__dirname, '..', '..', 'static', 'app.js'), 'utf8')];
+    for (const f of fs.readdirSync(jsDir).sort()) {
+        if (f.endsWith('.js')) parts.push(fs.readFileSync(path.join(jsDir, f), 'utf8'));
+    }
+    return parts.join('\n');
+}
+
 test('there are at least 8 song:* emit sites threaded through the helper', () => {
     // Sanity-check that the helper actually got wired everywhere. If the
     // count drops, someone removed an emit (regression) or refactored an
     // event away (intentional — this test then needs updating).
-    const src = fs.readFileSync(APP_JS, 'utf8');
+    const src = allFrontendSources();
     const matches = src.match(/(?:window\.feedBack|\w+)\.emit\(\s*['"]song:(play|pause|ended)['"][^)]*\)/g) || [];
     assert.ok(
         matches.length >= 8,

tests/js/song_restart.test.js

@@ -19,7 +19,9 @@ function buildSandbox({ loopA = null, loopB = null, isPlaying = false } = {}) {
     const sandbox = {
         loopA,
         loopB,
-        isPlaying,
+        // isPlaying moved onto the shared player-state container so a carved module can
+        // WRITE it (an imported binding is read-only). Same value, same assertions.
+        S: { isPlaying, lastAudioTime: 0 },
         __cancelCountInCalls: 0,
         __seekCalls: [],
         __startCountInCalls: [],
@@ -42,7 +44,7 @@ function buildSandbox({ loopA = null, loopB = null, isPlaying = false } = {}) {
         },
         __togglePlay() {
             sandbox.__togglePlayCalls++;
-            sandbox.isPlaying = true;
+            sandbox.S.isPlaying = true;
             return Promise.resolve();
         },
     };
@@ -53,7 +55,7 @@ function buildSandbox({ loopA = null, loopB = null, isPlaying = false } = {}) {
 function loadRestart(sandbox, src, { audioSeekImpl } = {}) {
     const restartSrc = extractFunction(src, 'async function restartCurrentSong(');
     const code = `
-        var isPlaying = ${sandbox.isPlaying};
+        var S = { isPlaying: ${sandbox.S.isPlaying}, lastAudioTime: 0 };
         function _cancelCountIn() { __cancelCountInCalls++; }
         async function _audioSeek(s, reason) {
             return (${audioSeekImpl || '__audioSeek'})(s, reason);

tests/js/song_seek.test.js

@@ -1,4 +1,4 @@
-// Verify static/app.js emits `song:seek` for every audio repositioning,
+// Verify static/js/transport.js emits `song:seek` for every audio repositioning,
 // with `{ from, to, reason }` payload. Plugins (notedetect detection-
 // suppression during seek transients) consume this contract.
 //
@@ -11,7 +11,7 @@ const fs = require('node:fs');
 const path = require('node:path');
 const vm = require('node:vm');
 
-const APP_JS = path.join(__dirname, '..', '..', 'static', 'app.js');
+const APP_JS = path.join(__dirname, '..', '..', 'static', 'js', 'transport.js');
 
 function extractFunction(src, signature) {
     const start = src.indexOf(signature);
@@ -77,7 +77,10 @@ function loadFunctions(sandbox, src) {
         // _audioSeek now syncs the jump-fix tracker so far seeks don't
         // trigger an immediate revert; declare it here so the sandbox
         // assignment lands on a real binding rather than an implicit global.
-        let lastAudioTime = 0;
+        // lastAudioTime moved onto the shared player-state container
+        // (static/js/player-state.js) so a carved module can WRITE it — an imported
+        // binding is read-only. The sliced code writes S.lastAudioTime now.
+        let S = { isPlaying: false, lastAudioTime: 0 };
         // _audioSeek wraps jucePlayer.seek in a timeout race; pull in the
         // helper + constant. Tests can override jucePlayer.seek to vary
         // behavior; the timeout (2 s) is well above any test setTimeout.
@@ -284,13 +287,26 @@ test('seekBy floors at zero (does not seek to negative time)', async () => {
     assert.equal(seek.detail.to, 0);
 });
 
+// CENSUS over the WHOLE frontend, not one file. This test counts call/emit sites, and the
+// carve keeps moving them between app.js and static/js/*.js — point it at a single file
+// and the count silently shrinks as code leaves, which reads as "someone deleted an emit"
+// (or, worse, passes while genuinely missing sites). Read every source that can hold one.
+function allFrontendSources() {
+    const jsDir = path.join(__dirname, '..', '..', 'static', 'js');
+    const parts = [fs.readFileSync(path.join(__dirname, '..', '..', 'static', 'app.js'), 'utf8')];
+    for (const f of fs.readdirSync(jsDir).sort()) {
+        if (f.endsWith('.js')) parts.push(fs.readFileSync(path.join(jsDir, f), 'utf8'));
+    }
+    return parts.join('\n');
+}
+
 test('every documented seek callsite passes a reason', () => {
     // Source-order assertion: every _audioSeek call outside the
     // implementation must pass a kebab-case reason string. Catches a
     // future contributor adding a new seek path without threading the
     // reason. Line-based — regex argument capture can't balance parens
     // through Math.max/_audioTime calls.
-    const src = fs.readFileSync(APP_JS, 'utf8');
+    const src = allFrontendSources();
     const fnSrc = extractFunction(src, 'async function _audioSeek(');
     const withoutImpl = src.replace(fnSrc, '');
     const callLines = withoutImpl.split('\n').filter((l) => /_audioSeek\(/.test(l));

tests/js/speed_reset.test.js

@@ -5,6 +5,9 @@ const path = require('node:path');
 const vm = require('node:vm');
 
 const APP_JS = path.join(__dirname, '..', '..', 'static', 'app.js');
+// The speed controls were carved out into static/js/player-controls.js (R3a); playSong,
+// which resets them on a new song, stayed in app.js. This test spans both.
+const CONTROLS_JS = path.join(__dirname, '..', '..', 'static', 'js', 'player-controls.js');
 
 function extractFunction(src, signature) {
     const start = src.indexOf(signature);
@@ -130,20 +133,30 @@ function extractConstLine(src, name) {
 
 function loadPlaySong(sandbox) {
     const src = fs.readFileSync(APP_JS, 'utf8');
-    const resetHelper = src.includes('function _resetPlaybackSpeedForNewSong')
-        ? extractFunction(src, 'function _resetPlaybackSpeedForNewSong')
+    // the module is ESM; the vm sandbox evaluates plain script text
+    const controls = fs.readFileSync(CONTROLS_JS, 'utf8').replace(/^export /gm, '');
+    const resetHelper = controls.includes('function _resetPlaybackSpeedForNewSong')
+        ? extractFunction(controls, 'function _resetPlaybackSpeedForNewSong')
         : '';
-    const speedPresetHelpers = src.includes('function _updateSpeedPresetButtons')
+    const speedPresetHelpers = controls.includes('function _updateSpeedPresetButtons')
         ? `
-        ${extractConstLine(src, 'SPEED_PRESET_PCTS')}
-        ${extractConstLine(src, 'SPEED_SNAP_THRESHOLD')}
-        ${extractFunction(src, 'function _speedPresetPctFromActive')}
-        ${extractFunction(src, 'function _updateSpeedPresetButtons')}
+        ${extractConstLine(controls, 'SPEED_PRESET_PCTS')}
+        ${extractConstLine(controls, 'SPEED_SNAP_THRESHOLD')}
+        ${extractFunction(controls, 'function _speedPresetPctFromActive')}
+        ${extractFunction(controls, 'function _updateSpeedPresetButtons')}
         `
         : '';
     const code = `
         var artAbortController = null;
-        var isPlaying = true;
+        // isPlaying moved onto the shared player-state container so a carved module can
+        // WRITE it (an imported binding is read-only). NB window.feedBack.isPlaying — the
+        // public mirror stubbed above — is a different thing and is unchanged.
+        var S = { isPlaying: true, lastAudioTime: 0 };
+        // The speed controls reach app.js through the host seam (static/js/host.js).
+        // Route it at the sandbox's EXISTING handleSliderInput spy — a fresh stub would
+        // swallow the call and the assertion below (which checks the slider was actually
+        // refreshed) would pass vacuously.
+        var host = { handleSliderInput: (el) => handleSliderInput(el) };
         var currentFilename = null;
         var _playerOriginScreen = null;
         var _pendingAutostart = false;
@@ -163,7 +176,7 @@ function loadPlaySong(sandbox) {
         function _scheduleSectionPracticeRetries() {}
         function loadSavedLoops() {}
         function _songEventPayload() { return { time: 7, audioT: 7, chartT: 7, perfNow: 7 }; }
-        ${extractFunction(src, 'function setSpeed')}
+        ${extractFunction(controls, 'function setSpeed')}
         ${speedPresetHelpers}
         ${resetHelper}
         ${extractFunction(src, 'async function playSong')}

tests/js/v3_library_refresh.test.js

@@ -16,7 +16,11 @@ const path = require('node:path');
 
 const root = path.join(__dirname, '..', '..');
 const SONGS = fs.readFileSync(path.join(root, 'static', 'v3', 'songs.js'), 'utf8');
-const APP = fs.readFileSync(path.join(root, 'static', 'app.js'), 'utf8');
+// The rescan path moved into ./static/js/library.js with the rest of the library (R3a).
+// Read BOTH: this asserts the emit exists SOMEWHERE in the app, and pinning it to one file
+// just means the test starts lying the next time the code moves.
+const APP = fs.readFileSync(path.join(root, 'static', 'app.js'), 'utf8')
+    + '\n' + fs.readFileSync(path.join(root, 'static', 'js', 'library.js'), 'utf8');
 
 test('app.js emits library:changed when a Settings rescan completes', () => {
     assert.match(APP, /emit\(\s*['"]library:changed['"]/,

tests/js/venue_crowd.test.js

@@ -0,0 +1,130 @@
+'use strict';
+
+const { test } = require('node:test');
+const assert = require('node:assert/strict');
+const fs = require('node:fs');
+const path = require('node:path');
+
+const crowd = require('../../static/v3/venue-crowd.js');
+
+const H3D_JS = path.join(__dirname, '..', '..', 'plugins', 'highway_3d', 'screen.js');
+const INDEX_HTML = path.join(__dirname, '..', '..', 'static', 'v3', 'index.html');
+const SCENE_JS = path.join(__dirname, '..', '..', 'static', 'v3', 'venue-scene-3d.js');
+
+test('perf state → crowd state mapping', () => {
+    assert.equal(crowd.crowdStateOfPerf('smoke'), 'bored');
+    assert.equal(crowd.crowdStateOfPerf('recovery'), 'bored');
+    assert.equal(crowd.crowdStateOfPerf('idle'), 'neutral');
+    assert.equal(crowd.crowdStateOfPerf('steady'), 'neutral');
+    assert.equal(crowd.crowdStateOfPerf('strong'), 'engaged');
+    assert.equal(crowd.crowdStateOfPerf('fire'), 'ecstatic');
+    assert.equal(crowd.crowdStateOfPerf('FIRE'), 'ecstatic');
+    assert.equal(crowd.crowdStateOfPerf('bogus'), 'neutral');
+    assert.equal(crowd.crowdStateOfPerf(undefined), 'neutral');
+});
+
+test('machine: target must be stable for STABLE_MS before committing', () => {
+    const m = crowd.createCrowdMachine();
+    assert.equal(m.current, 'neutral');
+    assert.equal(m.update('fire', 0), null);
+    assert.equal(m.update('fire', crowd.STABLE_MS - 1), null);
+    assert.equal(m.update('fire', crowd.STABLE_MS), 'ecstatic');
+    assert.equal(m.current, 'ecstatic');
+});
+
+test('machine: flapping target restarts the stability window', () => {
+    const m = crowd.createCrowdMachine();
+    m.update('fire', 0);
+    // Target changes → candidate resets.
+    m.update('strong', 1000);
+    assert.equal(m.update('strong', 1000 + crowd.STABLE_MS - 1), null);
+    assert.equal(m.update('strong', 1000 + crowd.STABLE_MS), 'engaged');
+});
+
+test('machine: returning to current state clears the candidate', () => {
+    const m = crowd.createCrowdMachine();
+    m.update('fire', 0);
+    m.update('steady', 1000); // back to neutral (current) — candidate dropped
+    // 'fire' again must wait a full stability window from scratch.
+    assert.equal(m.update('fire', 2000), null);
+    assert.equal(m.update('fire', 2000 + crowd.STABLE_MS - 1), null);
+    assert.equal(m.update('fire', 2000 + crowd.STABLE_MS), 'ecstatic');
+});
+
+test('machine: DWELL_MS enforced between switches', () => {
+    const m = crowd.createCrowdMachine();
+    m.update('fire', 0);
+    assert.equal(m.update('fire', crowd.STABLE_MS), 'ecstatic'); // switch at t=3000
+    const t = crowd.STABLE_MS;
+    // Immediately drop to smoke: stable window passes but dwell hasn't.
+    m.update('smoke', t + 1);
+    assert.equal(m.update('smoke', t + 1 + crowd.STABLE_MS), null);
+    // After the dwell expires the pending candidate commits.
+    assert.equal(m.update('smoke', t + crowd.DWELL_MS), 'bored');
+});
+
+test('machine: multi-step jumps allowed (bored → ecstatic)', () => {
+    const m = crowd.createCrowdMachine();
+    m.update('smoke', 0);
+    assert.equal(m.update('smoke', crowd.STABLE_MS), 'bored');
+    const t = crowd.DWELL_MS + 1000;
+    m.update('fire', t);
+    assert.equal(m.update('fire', t + crowd.STABLE_MS), 'ecstatic');
+});
+
+test('stingerForStreak fires on rising milestone crossings only', () => {
+    assert.equal(crowd.stingerForStreak(24, 25), 'cheer');
+    assert.equal(crowd.stingerForStreak(0, 100), 'cheer');
+    assert.equal(crowd.stingerForStreak(25, 26), null);
+    assert.equal(crowd.stingerForStreak(50, 50), null);
+    assert.equal(crowd.stingerForStreak(30, 0), null); // streak reset
+});
+
+test('stingerForAccuracy thresholds', () => {
+    assert.equal(crowd.stingerForAccuracy(95), 'cheer');
+    assert.equal(crowd.stingerForAccuracy(90), 'cheer');
+    assert.equal(crowd.stingerForAccuracy(80), 'clap');
+    assert.equal(crowd.stingerForAccuracy(75), 'clap');
+    assert.equal(crowd.stingerForAccuracy(60), null);
+    assert.equal(crowd.stingerForAccuracy('nope'), null);
+    assert.equal(crowd.stingerForAccuracy(undefined), null);
+});
+
+test('normalizeManifest requires all four loops, resolves base', () => {
+    assert.equal(crowd.normalizeManifest(null), null);
+    assert.equal(crowd.normalizeManifest({}), null);
+    assert.equal(crowd.normalizeManifest({ loops: { bored: 'b.mp4' } }), null);
+    const m = crowd.normalizeManifest({
+        base: '/api/plugins/career/venues/bar/',
+        loops: { bored: 'bored.mp4', neutral: 'neutral.mp4', engaged: 'engaged.mp4', ecstatic: 'ecstatic.mp4' },
+        stingers: { cheer: 'cheer.mp4' },
+    });
+    assert.equal(m.loops.ecstatic, '/api/plugins/career/venues/bar/ecstatic.mp4');
+    assert.equal(m.stingers.cheer, '/api/plugins/career/venues/bar/cheer.mp4');
+    assert.equal(m.stingers.clap, '');
+});
+
+test('highway_3d exposes the crowd backdrop globals', () => {
+    const src = fs.readFileSync(H3D_JS, 'utf8');
+    assert.match(src, /window\.h3dVenueBackdropSetVideo\s*=/);
+    assert.match(src, /window\.h3dVenueBackdropSetMix\s*=/);
+    // The venue style must own crowd plane teardown (VideoTexture dispose).
+    assert.match(src, /_venueCrowdVideos/);
+    assert.match(src, /_venueCrowdMix/);
+});
+
+test('index.html loads venue-crowd.js deferred, after venue-scene-3d.js', () => {
+    const html = fs.readFileSync(INDEX_HTML, 'utf8');
+    const crowdIdx = html.indexOf('/static/v3/venue-crowd.js');
+    const sceneIdx = html.indexOf('/static/v3/venue-scene-3d.js');
+    assert.ok(crowdIdx > 0, 'venue-crowd.js script tag missing');
+    assert.ok(crowdIdx > sceneIdx, 'venue-crowd.js must load after venue-scene-3d.js');
+    assert.match(html, /<script defer src="\/static\/v3\/venue-crowd\.js"><\/script>/);
+});
+
+test('venue-scene-3d activates/deactivates the crowd layer', () => {
+    const src = fs.readFileSync(SCENE_JS, 'utf8');
+    assert.match(src, /syncCrowd\(true\)/);
+    assert.match(src, /syncCrowd\(false\)/);
+    assert.match(src, /v3VenueCrowd/);
+});

tests/test_builtin_diagnostic_seed.py

@@ -7,6 +7,7 @@ import os
 import sys
 import time
 
+import builtin_content
 import pytest
 
 
@@ -23,13 +24,13 @@ def test_seed_creates_builtin_diagnostic_sloppak(tmp_path, server_mod):
     """First seed copies the bundled sloppak into diagnostics-builtin/."""
     dlc = tmp_path / "dlc"
     dlc.mkdir()
-    source = server_mod._feedBack_server_root() / server_mod._BUILTIN_DIAGNOSTIC_SOURCES[0][1]
+    source = server_mod._feedBack_server_root() / builtin_content.BUILTIN_DIAGNOSTIC_SOURCES[0][1]
     if not source.is_file():
         pytest.skip(f"source sloppak not present in checkout: {source}")
 
-    server_mod._seed_builtin_diagnostic_sloppaks(dlc)
+    builtin_content.seed_builtin_diagnostic_sloppaks(server_mod._feedBack_server_root(), dlc)
 
-    dest = dlc / server_mod._BUILTIN_DIAGNOSTIC_SUBDIR / server_mod._BUILTIN_DIAGNOSTIC_SOURCES[0][0]
+    dest = dlc / builtin_content.BUILTIN_DIAGNOSTIC_SUBDIR / builtin_content.BUILTIN_DIAGNOSTIC_SOURCES[0][0]
     assert dest.is_file()
     assert dest.stat().st_size == source.stat().st_size
 
@@ -38,16 +39,16 @@ def test_seed_is_idempotent_when_destination_exists(tmp_path, server_mod):
     """Second seed leaves an up-to-date destination unchanged."""
     dlc = tmp_path / "dlc"
     dlc.mkdir()
-    source = server_mod._feedBack_server_root() / server_mod._BUILTIN_DIAGNOSTIC_SOURCES[0][1]
+    source = server_mod._feedBack_server_root() / builtin_content.BUILTIN_DIAGNOSTIC_SOURCES[0][1]
     if not source.is_file():
         pytest.skip(f"source sloppak not present in checkout: {source}")
 
-    server_mod._seed_builtin_diagnostic_sloppaks(dlc)
-    dest = dlc / server_mod._BUILTIN_DIAGNOSTIC_SUBDIR / server_mod._BUILTIN_DIAGNOSTIC_SOURCES[0][0]
+    builtin_content.seed_builtin_diagnostic_sloppaks(server_mod._feedBack_server_root(), dlc)
+    dest = dlc / builtin_content.BUILTIN_DIAGNOSTIC_SUBDIR / builtin_content.BUILTIN_DIAGNOSTIC_SOURCES[0][0]
     first_mtime = dest.stat().st_mtime_ns
     first_size = dest.stat().st_size
 
-    server_mod._seed_builtin_diagnostic_sloppaks(dlc)
+    builtin_content.seed_builtin_diagnostic_sloppaks(server_mod._feedBack_server_root(), dlc)
 
     assert dest.stat().st_mtime_ns == first_mtime
     assert dest.stat().st_size == first_size
@@ -57,18 +58,18 @@ def test_seed_skips_when_destination_is_newer(tmp_path, server_mod):
     """An existing newer destination is not overwritten."""
     dlc = tmp_path / "dlc"
     dlc.mkdir()
-    source = server_mod._feedBack_server_root() / server_mod._BUILTIN_DIAGNOSTIC_SOURCES[0][1]
+    source = server_mod._feedBack_server_root() / builtin_content.BUILTIN_DIAGNOSTIC_SOURCES[0][1]
     if not source.is_file():
         pytest.skip(f"source sloppak not present in checkout: {source}")
-    dest_dir = dlc / server_mod._BUILTIN_DIAGNOSTIC_SUBDIR
+    dest_dir = dlc / builtin_content.BUILTIN_DIAGNOSTIC_SUBDIR
     dest_dir.mkdir(parents=True)
-    dest_name = server_mod._BUILTIN_DIAGNOSTIC_SOURCES[0][0]
+    dest_name = builtin_content.BUILTIN_DIAGNOSTIC_SOURCES[0][0]
     dest = dest_dir / dest_name
     dest.write_bytes(b"user-owned diagnostic copy")
     future = time.time() + 3600
     os.utime(dest, (future, future))
 
-    server_mod._seed_builtin_diagnostic_sloppaks(dlc)
+    builtin_content.seed_builtin_diagnostic_sloppaks(server_mod._feedBack_server_root(), dlc)
 
     assert dest.read_bytes() == b"user-owned diagnostic copy"
 
@@ -77,18 +78,18 @@ def test_seed_refuses_to_follow_symlink_destination(tmp_path, server_mod):
     """A symlink at the destination is skipped, not written through."""
     dlc = tmp_path / "dlc"
     dlc.mkdir()
-    source = server_mod._feedBack_server_root() / server_mod._BUILTIN_DIAGNOSTIC_SOURCES[0][1]
+    source = server_mod._feedBack_server_root() / builtin_content.BUILTIN_DIAGNOSTIC_SOURCES[0][1]
     if not source.is_file():
         pytest.skip(f"source sloppak not present in checkout: {source}")
 
     outside = tmp_path / "outside.txt"
     outside.write_bytes(b"do not overwrite me")
-    dest_dir = dlc / server_mod._BUILTIN_DIAGNOSTIC_SUBDIR
+    dest_dir = dlc / builtin_content.BUILTIN_DIAGNOSTIC_SUBDIR
     dest_dir.mkdir(parents=True)
-    dest = dest_dir / server_mod._BUILTIN_DIAGNOSTIC_SOURCES[0][0]
+    dest = dest_dir / builtin_content.BUILTIN_DIAGNOSTIC_SOURCES[0][0]
     dest.symlink_to(outside)
 
-    server_mod._seed_builtin_diagnostic_sloppaks(dlc)
+    builtin_content.seed_builtin_diagnostic_sloppaks(server_mod._feedBack_server_root(), dlc)
 
     # The symlink target must be untouched and the link left as-is.
     assert outside.read_bytes() == b"do not overwrite me"
@@ -101,11 +102,11 @@ def test_seed_refuses_symlinked_seed_directory(tmp_path, server_mod):
     dlc.mkdir()
     outside_dir = tmp_path / "outside_dir"
     outside_dir.mkdir()
-    (dlc / server_mod._BUILTIN_DIAGNOSTIC_SUBDIR).symlink_to(
+    (dlc / builtin_content.BUILTIN_DIAGNOSTIC_SUBDIR).symlink_to(
         outside_dir, target_is_directory=True
     )
 
-    server_mod._seed_builtin_diagnostic_sloppaks(dlc)
+    builtin_content.seed_builtin_diagnostic_sloppaks(server_mod._feedBack_server_root(), dlc)
 
     # Nothing was written through the directory symlink into the link target.
     assert list(outside_dir.iterdir()) == []
@@ -116,11 +117,11 @@ def test_seed_missing_source_does_not_crash(tmp_path, server_mod, monkeypatch):
     dlc = tmp_path / "dlc"
     dlc.mkdir()
     monkeypatch.setattr(
-        server_mod,
-        "_BUILTIN_DIAGNOSTIC_SOURCES",
+        builtin_content,
+        "BUILTIN_DIAGNOSTIC_SOURCES",
         [("missing.sloppak", "docs/diagnostics/does-not-exist.sloppak")],
     )
 
-    server_mod._seed_builtin_diagnostic_sloppaks(dlc)
+    builtin_content.seed_builtin_diagnostic_sloppaks(server_mod._feedBack_server_root(), dlc)
 
-    assert not (dlc / server_mod._BUILTIN_DIAGNOSTIC_SUBDIR / "missing.sloppak").exists()
+    assert not (dlc / builtin_content.BUILTIN_DIAGNOSTIC_SUBDIR / "missing.sloppak").exists()

tests/test_builtin_starter_seed.py

@@ -5,6 +5,7 @@ from __future__ import annotations
 import importlib
 import sys
 
+import builtin_content
 import pytest
 
 
@@ -21,15 +22,15 @@ def server_mod(tmp_path, monkeypatch, isolate_logging):
 def _source(server_mod):
     return (
         server_mod._feedBack_server_root()
-        / server_mod._BUILTIN_STARTER_SOURCES[0][1]
+        / builtin_content.BUILTIN_STARTER_SOURCES[0][1]
     )
 
 
 def _dest(server_mod, dlc):
     return (
         dlc
-        / server_mod._BUILTIN_STARTER_SUBDIR
-        / server_mod._BUILTIN_STARTER_SOURCES[0][0]
+        / builtin_content.BUILTIN_STARTER_SUBDIR
+        / builtin_content.BUILTIN_STARTER_SOURCES[0][0]
     )
 
 
@@ -41,12 +42,12 @@ def test_seed_creates_starter_content_and_marker(tmp_path, server_mod):
     if not source.is_file():
         pytest.skip(f"starter source not present in checkout: {source}")
 
-    server_mod._seed_builtin_starter_content(dlc)
+    builtin_content.seed_builtin_starter_content(server_mod._feedBack_server_root(), dlc)
 
     dest = _dest(server_mod, dlc)
     assert dest.is_file()
     assert dest.stat().st_size == source.stat().st_size
-    assert (server_mod.CONFIG_DIR / server_mod._STARTER_SEED_MARKER).is_file()
+    assert (server_mod.CONFIG_DIR / builtin_content.STARTER_SEED_MARKER).is_file()
 
 
 def test_seed_preserves_source_mtime(tmp_path, server_mod):
@@ -58,7 +59,7 @@ def test_seed_preserves_source_mtime(tmp_path, server_mod):
     if not source.is_file():
         pytest.skip(f"starter source not present in checkout: {source}")
 
-    server_mod._seed_builtin_starter_content(dlc)
+    builtin_content.seed_builtin_starter_content(server_mod._feedBack_server_root(), dlc)
 
     assert _dest(server_mod, dlc).stat().st_mtime_ns == source.stat().st_mtime_ns
 
@@ -78,7 +79,7 @@ def test_seed_runs_only_once_and_respects_deletion(tmp_path, server_mod):
     if not source.is_file():
         pytest.skip(f"starter source not present in checkout: {source}")
 
-    server_mod._seed_builtin_starter_content(dlc)
+    builtin_content.seed_builtin_starter_content(server_mod._feedBack_server_root(), dlc)
     dest = _dest(server_mod, dlc)
     assert dest.is_file()
 
@@ -86,7 +87,7 @@ def test_seed_runs_only_once_and_respects_deletion(tmp_path, server_mod):
     dest.unlink()
 
     # A subsequent launch must not re-seed it.
-    server_mod._seed_builtin_starter_content(dlc)
+    builtin_content.seed_builtin_starter_content(server_mod._feedBack_server_root(), dlc)
     assert not dest.exists()
 
 
@@ -98,15 +99,15 @@ def test_seed_deferred_until_dlc_configured(tmp_path, server_mod):
         pytest.skip(f"starter source not present in checkout: {source}")
 
     # dlc is None and DLC_DIR unset -> _get_dlc_dir() returns None.
-    server_mod._seed_builtin_starter_content(None)
-    assert not (server_mod.CONFIG_DIR / server_mod._STARTER_SEED_MARKER).exists()
+    builtin_content.seed_builtin_starter_content(server_mod._feedBack_server_root(), None)
+    assert not (server_mod.CONFIG_DIR / builtin_content.STARTER_SEED_MARKER).exists()
 
     # Now a DLC is configured: the deferred seed runs.
     dlc = tmp_path / "dlc"
     dlc.mkdir()
-    server_mod._seed_builtin_starter_content(dlc)
+    builtin_content.seed_builtin_starter_content(server_mod._feedBack_server_root(), dlc)
     assert _dest(server_mod, dlc).is_file()
-    assert (server_mod.CONFIG_DIR / server_mod._STARTER_SEED_MARKER).is_file()
+    assert (server_mod.CONFIG_DIR / builtin_content.STARTER_SEED_MARKER).is_file()
 
 
 def test_seed_refuses_symlinked_seed_directory(tmp_path, server_mod):
@@ -119,13 +120,13 @@ def test_seed_refuses_symlinked_seed_directory(tmp_path, server_mod):
 
     outside_dir = tmp_path / "outside"
     outside_dir.mkdir()
-    (dlc / server_mod._BUILTIN_STARTER_SUBDIR).symlink_to(outside_dir)
+    (dlc / builtin_content.BUILTIN_STARTER_SUBDIR).symlink_to(outside_dir)
 
-    server_mod._seed_builtin_starter_content(dlc)
+    builtin_content.seed_builtin_starter_content(server_mod._feedBack_server_root(), dlc)
 
     assert list(outside_dir.iterdir()) == []
     # An incomplete seed must NOT write the marker, so a later launch retries.
-    assert not (server_mod.CONFIG_DIR / server_mod._STARTER_SEED_MARKER).exists()
+    assert not (server_mod.CONFIG_DIR / builtin_content.STARTER_SEED_MARKER).exists()
 
 
 def test_seed_never_overwrites_an_existing_user_file(tmp_path, server_mod):
@@ -140,11 +141,11 @@ def test_seed_never_overwrites_an_existing_user_file(tmp_path, server_mod):
     dest.write_bytes(b"user's own edited pack")
     _os.utime(dest, (1_000_000, 1_000_000))  # far older than the bundled source
 
-    server_mod._seed_builtin_starter_content(dlc)
+    builtin_content.seed_builtin_starter_content(server_mod._feedBack_server_root(), dlc)
 
     assert dest.read_bytes() == b"user's own edited pack"  # untouched
     # counted as already-present, so the one-time seed considers itself done
-    assert (server_mod.CONFIG_DIR / server_mod._STARTER_SEED_MARKER).is_file()
+    assert (server_mod.CONFIG_DIR / builtin_content.STARTER_SEED_MARKER).is_file()
 
 
 def test_seed_does_not_mark_when_destination_is_a_directory(tmp_path, server_mod):
@@ -160,10 +161,10 @@ def test_seed_does_not_mark_when_destination_is_a_directory(tmp_path, server_mod
     bogus.parent.mkdir(parents=True, exist_ok=True)
     bogus.mkdir()  # user (or junk) placed a directory where the pack goes
 
-    server_mod._seed_builtin_starter_content(dlc)
+    builtin_content.seed_builtin_starter_content(server_mod._feedBack_server_root(), dlc)
 
     assert bogus.is_dir()  # untouched
-    assert not (server_mod.CONFIG_DIR / server_mod._STARTER_SEED_MARKER).exists()
+    assert not (server_mod.CONFIG_DIR / builtin_content.STARTER_SEED_MARKER).exists()
 
 
 def test_seed_does_not_mark_when_source_missing(tmp_path, server_mod, monkeypatch):
@@ -172,15 +173,15 @@ def test_seed_does_not_mark_when_source_missing(tmp_path, server_mod, monkeypatc
     dlc = tmp_path / "dlc"
     dlc.mkdir()
     monkeypatch.setattr(
-        server_mod,
-        "_BUILTIN_STARTER_SOURCES",
+        builtin_content,
+        "BUILTIN_STARTER_SOURCES",
         [("missing.feedpak", "content/starter/does-not-exist.feedpak")],
     )
 
-    server_mod._seed_builtin_starter_content(dlc)
+    builtin_content.seed_builtin_starter_content(server_mod._feedBack_server_root(), dlc)
 
-    assert not (dlc / server_mod._BUILTIN_STARTER_SUBDIR / "missing.feedpak").exists()
-    assert not (server_mod.CONFIG_DIR / server_mod._STARTER_SEED_MARKER).exists()
+    assert not (dlc / builtin_content.BUILTIN_STARTER_SUBDIR / "missing.feedpak").exists()
+    assert not (server_mod.CONFIG_DIR / builtin_content.STARTER_SEED_MARKER).exists()
 
 
 def test_every_starter_source_file_is_present(server_mod):
@@ -190,7 +191,7 @@ def test_every_starter_source_file_is_present(server_mod):
     the checkout is clean, so "on disk" == committed."""
     root = server_mod._feedBack_server_root()
     missing = [
-        rel for _, rel in server_mod._BUILTIN_STARTER_SOURCES
+        rel for _, rel in builtin_content.BUILTIN_STARTER_SOURCES
         if not (root / rel).is_file()
     ]
     assert not missing, f"listed starter sources missing on disk: {missing}"
@@ -199,18 +200,18 @@ def test_every_starter_source_file_is_present(server_mod):
 def test_seed_lands_every_listed_starter_pack(tmp_path, server_mod):
     """A real seed run copies every listed pack into starter/ and marks done."""
     root = server_mod._feedBack_server_root()
-    for _, rel in server_mod._BUILTIN_STARTER_SOURCES:
+    for _, rel in builtin_content.BUILTIN_STARTER_SOURCES:
         if not (root / rel).is_file():
             pytest.skip(f"starter source not present in checkout: {rel}")
 
     dlc = tmp_path / "dlc"
     dlc.mkdir()
-    server_mod._seed_builtin_starter_content(dlc)
+    builtin_content.seed_builtin_starter_content(server_mod._feedBack_server_root(), dlc)
 
-    for dest_name, _ in server_mod._BUILTIN_STARTER_SOURCES:
-        dest = dlc / server_mod._BUILTIN_STARTER_SUBDIR / dest_name
+    for dest_name, _ in builtin_content.BUILTIN_STARTER_SOURCES:
+        dest = dlc / builtin_content.BUILTIN_STARTER_SUBDIR / dest_name
         assert dest.is_file(), f"pack not seeded: {dest_name}"
-    assert (server_mod.CONFIG_DIR / server_mod._STARTER_SEED_MARKER).is_file()
+    assert (server_mod.CONFIG_DIR / builtin_content.STARTER_SEED_MARKER).is_file()
 
 
 def test_no_unlisted_starter_pack_on_disk(server_mod):
@@ -220,7 +221,7 @@ def test_no_unlisted_starter_pack_on_disk(server_mod):
     main before being wired up. In CI the checkout is clean, so this flags any
     stray/committed pack that isn't listed."""
     root = server_mod._feedBack_server_root()
-    listed = {rel for _, rel in server_mod._BUILTIN_STARTER_SOURCES}
+    listed = {rel for _, rel in builtin_content.BUILTIN_STARTER_SOURCES}
     if not listed:
         pytest.skip("no starter sources declared")
     content_dir = (root / next(iter(listed))).parent  # all sources share this dir

tests/test_plugin_context_contract.py

@@ -0,0 +1,205 @@
+"""The plugin context is a THIRD-PARTY CONTRACT. Pin it.
+
+`context` is handed to every plugin's `setup()`. Plugins — including ones we don't ship
+and can't grep — read keys out of it and hold the callables as live references. Issue #48
+flagged this while planning the server.py split and asked for exactly this assertion:
+
+    "Plugin context[...] are passed as live references into already-loaded plugins.
+     Refactoring must preserve the exact callables — moving them to a new module is
+     fine, but renaming or wrapping them breaks third-party plugins. We'd want a
+     'plugin context unchanged' assertion in CI."
+
+It doesn't exist yet, and server.py is about to be carved apart around the code that
+builds it. This is the guard that makes the carve safe: a key silently dropped or
+renamed by a move is invisible to every other test in the suite (nothing in-tree reads
+most of these) and would break plugins at runtime, in the field.
+
+Same lesson the frontend carve learned the hard way: a contract that only external code
+reads cannot be found by a call-graph scan, so it has to be pinned by name.
+
+WHY A LITERAL LIST AND NOT A DERIVED ONE. Deriving the expected set from the source would
+assert the code equals itself. The whole point is that a human has to look at a diff and
+consciously agree to change the contract.
+"""
+
+import ast
+from pathlib import Path
+
+import pytest
+
+SERVER_PY = Path(__file__).resolve().parents[1] / "server.py"
+PLUGINS_PY = Path(__file__).resolve().parents[1] / "plugins" / "__init__.py"
+
+# The keys server.py puts in the shared context handed to register_plugin_api().
+BASE_CONTEXT_KEYS = {
+    "config_dir",
+    "get_dlc_dir",
+    "extract_meta",
+    "meta_db",
+    "get_scan_status",
+    "get_art_cache_dir",
+    "library_providers",
+    "register_library_provider",
+    "unregister_library_provider",
+    "register_tuning_provider",
+    "unregister_tuning_provider",
+    "get_sloppak_cache_dir",
+    "register_demo_janitor_hook",
+    "award_xp",
+    "get_xp_progress",
+    "seed_xp",
+    "reset_xp",
+    "record_progression_event",
+}
+
+# Added PER PLUGIN by plugins/__init__.py on top of the base — so the surface a plugin
+# actually sees is the union. Real shipped plugins read `log` and `load_sibling`, and
+# neither is in server.py's dict; a test that pinned only the base would miss them.
+PER_PLUGIN_KEYS = {"load_sibling", "log"}
+
+FULL_CONTEXT = BASE_CONTEXT_KEYS | PER_PLUGIN_KEYS
+
+
+def _plugin_context_keys() -> set:
+    """The literal keys of server.py's `plugin_context = {...}`, read from the AST.
+
+    AST, not a regex: the dict spans ~40 lines and is dense with comments, lambdas and
+    nested calls, and the values contain braces of their own.
+    """
+    tree = ast.parse(SERVER_PY.read_text(encoding="utf-8"))
+    for node in ast.walk(tree):
+        if (
+            isinstance(node, ast.Assign)
+            and node.targets
+            and isinstance(node.targets[0], ast.Name)
+            and node.targets[0].id == "plugin_context"
+            and isinstance(node.value, ast.Dict)
+        ):
+            keys = set()
+            for k in node.value.keys:
+                assert isinstance(k, ast.Constant), (
+                    "plugin_context must be built from literal string keys — a computed "
+                    "key makes this contract un-reviewable"
+                )
+                keys.add(k.value)
+            return keys
+    pytest.fail(
+        "server.py no longer builds a literal `plugin_context = {...}` dict. If it moved "
+        "to another module, point this test at that module — do NOT delete it."
+    )
+
+
+def test_plugin_context_keys_are_exactly_the_pinned_contract():
+    actual = _plugin_context_keys()
+
+    missing = BASE_CONTEXT_KEYS - actual
+    added = actual - BASE_CONTEXT_KEYS
+
+    assert not missing, (
+        f"plugin_context lost {sorted(missing)}. Every one of these is read by plugins we "
+        "do not control and cannot grep. Dropping one breaks them at runtime, in the "
+        "field, with nothing else in this suite failing."
+    )
+    assert not added, (
+        f"plugin_context gained {sorted(added)}. That's fine — but it is a PUBLIC API "
+        "addition, so add the key to BASE_CONTEXT_KEYS here deliberately, and document it "
+        "in docs/. This test exists to make that a conscious act rather than a side effect."
+    )
+
+
+def test_per_plugin_keys_are_still_layered_on_top():
+    """`log` and `load_sibling` are added per-plugin in plugins/__init__.py, not by
+    server.py — so they're invisible to the check above. Real plugins read both."""
+    src = PLUGINS_PY.read_text(encoding="utf-8")
+    for key in sorted(PER_PLUGIN_KEYS):
+        assert f'plugin_context["{key}"]' in src, (
+            f"plugins/__init__.py no longer sets plugin_context[{key!r}] — shipped plugins "
+            "read it"
+        )
+
+
+def test_context_values_reach_a_REAL_plugin_by_identity(tmp_path, reset_plugin_state):
+    """The contract is CALLABLE IDENTITY, not just key names.
+
+    A carve that moves these into a module and re-exports them through a wrapper (a
+    property, a functools.partial, a lazily-bound getter) keeps every key name intact and
+    STILL breaks plugins that stored the reference at setup() time.
+
+    Codex [P2] on the first cut of this test, and it was right: I originally built a dict
+    locally and called setup() on it, which asserts `dict(x)['k'] is x['k']` — trivially
+    true, and blind to everything plugins/__init__.py does. It has to go through the REAL
+    loader, because the real loader is exactly what copies and re-binds the context.
+
+    (That is not hypothetical: `register_library_provider` IS deliberately wrapped by the
+    loader, per-plugin, to force owner attribution. Pinned below so the one intentional
+    exception can't quietly become two.)
+    """
+    from fastapi import FastAPI
+
+    # reset_plugin_state (tests/conftest.py) is the ONLY safe way to drive the real
+    # load_plugins(): it also mutates sys.path, sys.modules and PENDING_PLUGINS, and a
+    # hand-rolled partial restore makes the suite order- and environment-dependent.
+    # Codex [P2] on the first cut of this, and it was right.
+    plugins_mod = reset_plugin_state
+
+    plugin_dir = tmp_path / "ctxprobe"
+    plugin_dir.mkdir()
+    (plugin_dir / "plugin.json").write_text(
+        '{"id": "ctxprobe", "name": "ctx probe", "routes": "routes.py"}'
+    )
+    # A backend plugin's entry point is routes.py's `setup(app, ctx)` — the same shape
+    # tests/test_plugins.py::_make_plugin uses. The probe hands the context BACK through a
+    # sink in the context itself: importing the probe module by name does not work (the
+    # loader namespaces plugin modules), and a file/JSON channel would lose the object
+    # IDENTITY that is the entire point of this test.
+    (plugin_dir / "routes.py").write_text(
+        "def setup(app, ctx):\n"
+        "    ctx['_probe_sink'].append(ctx)\n"
+    )
+
+    sentinel_db = object()
+
+    def sentinel_extract(_p):
+        return {}
+
+    def sentinel_register_library_provider(provider, *a, **kw):
+        return None
+
+    sink = []
+    context = {
+        "_probe_sink": sink,
+        "meta_db": sentinel_db,
+        "extract_meta": sentinel_extract,
+        "config_dir": tmp_path,
+        "register_library_provider": sentinel_register_library_provider,
+    }
+
+    app = FastAPI()
+    saved_dir = plugins_mod.PLUGINS_DIR
+    plugins_mod.PLUGINS_DIR = tmp_path
+    try:
+        plugins_mod.load_plugins(app, context)
+    finally:
+        plugins_mod.PLUGINS_DIR = saved_dir
+
+    assert sink, "the probe plugin's setup() never ran — the harness is not exercising the loader"
+    seen = sink[0]
+
+    assert seen["meta_db"] is sentinel_db, "meta_db must reach a real plugin BY IDENTITY"
+    assert seen["extract_meta"] is sentinel_extract, (
+        "extract_meta must reach a real plugin BY IDENTITY — wrapping it (partial, "
+        "property, re-binding getter) breaks plugins that stored the reference at setup()"
+    )
+    assert seen["config_dir"] is context["config_dir"]
+
+    # The loader adds these per-plugin; shipped plugins read both.
+    assert callable(seen["load_sibling"])
+    assert seen["log"].name == "feedBack.plugin.ctxprobe"
+
+    # THE ONE DELIBERATE WRAPPER. register_library_provider is scoped per-plugin so a
+    # plugin cannot forge owner attribution and impersonate another. Pinned so that the
+    # single intentional exception to identity cannot quietly become two.
+    assert seen["register_library_provider"] is not sentinel_register_library_provider, (
+        "register_library_provider is supposed to be wrapped per-plugin for owner "
+        "attribution — if that wrapper is gone, a plugin can impersonate another"
+    )

tests/test_plugin_runtime_idempotence.py

@@ -186,10 +186,16 @@ def test_app_event_bus_dispatches_locally_and_preserves_juce_stop_state():
     source = (ROOT / "static" / "app.js").read_text(encoding="utf-8")
 
     assert "this.dispatchEvent(new CustomEvent(event, { detail }))" in source
-    assert "const hadPlayableSong = !!audio.src || !!window._juceAudioUrl || isPlaying" in source
-    assert "sm.emit('song:resume', payload)" in source
+    # `isPlaying` moved onto the shared player-state container (static/js/player-state.js)
+    # so a carved module can WRITE it — an imported binding is read-only.
+    assert "const hadPlayableSong = !!audio.src || !!window._juceAudioUrl || S.isPlaying" in source
     assert "window.feedBack.emit('song:resume', payload)" in source
 
+    # The JUCE audio-element shim — which re-emits song:resume through the session
+    # manager when JUCE owns the transport — was carved out into its own module (R3a).
+    juce = (ROOT / "static" / "js" / "juce-audio.js").read_text(encoding="utf-8")
+    assert "sm.emit('song:resume', payload)" in juce
+
 
 def test_nam_and_stems_use_owner_claim_dispatch_semantics():
     nam_source = _sibling_text("feedBack-plugin-nam-tone", "screen.js", "NAM_STEM_CLAIM_ID = 'nam.amp-active'")

tests/test_plugin_src_route.py

@@ -138,3 +138,119 @@ def test_unready_plugin_src_is_404(client):
     c, _ = client
     plugins.LOADED_PLUGINS[0]["status"] = "installing"
     assert c.get(f"/api/plugins/{PLUGIN_ID}/src/main.js").status_code == 404
+
+
+# ── #879: the /g/<token>/ generation prefix ────────────────────────────────────
+#
+# A plugin ROLLBACK must actually re-evaluate a module plugin. ES modules are
+# evaluated once per URL per document, so re-inserting a <script type="module">
+# whose src the module map has already seen fires `load` without re-running the
+# body. Busting the ENTRY url alone does not help — screen.js is a one-line
+# `import './src/main.js'`, and a relative specifier resolves against the base URL
+# with the QUERY DROPPED, so a ?v= token never reaches the graph.
+#
+# Hence a token in the PATH: every relative import inherits it, at every depth,
+# with no import-specifier rewriting. These routes must serve the SAME bytes and
+# keep the SAME containment.
+
+def test_generation_prefix_serves_identical_screen_js(client):
+    c, _ = client
+    plain = c.get(f"/api/plugins/{PLUGIN_ID}/screen.js")
+    gen = c.get(f"/api/plugins/{PLUGIN_ID}/g/7/screen.js")
+    assert gen.status_code == 200
+    assert gen.content == plain.content
+    assert "import './src/main.js'" in gen.text
+
+
+def test_generation_prefix_serves_the_whole_module_graph(client):
+    """The point of the path token: a relative import from a /g/7/ entry resolves
+    to a /g/7/ URL, so the graph is fetched fresh — not just the entry."""
+    c, _ = client
+    main = c.get(f"/api/plugins/{PLUGIN_ID}/g/7/src/main.js")
+    assert main.status_code == 200
+    assert main.text == c.get(f"/api/plugins/{PLUGIN_ID}/src/main.js").text
+    # and one level deeper, which is where a query-string token would already have
+    # been lost twice over
+    nested = c.get(f"/api/plugins/{PLUGIN_ID}/g/7/src/util/x.js")
+    assert nested.status_code == 200
+    assert "export const x = 42" in nested.text
+
+
+def test_generation_token_is_opaque(client):
+    """Any token serves the same bytes — it exists only to vary the URL."""
+    c, _ = client
+    a = c.get(f"/api/plugins/{PLUGIN_ID}/g/1/src/main.js")
+    b = c.get(f"/api/plugins/{PLUGIN_ID}/g/999999/src/main.js")
+    assert a.status_code == b.status_code == 200
+    assert a.text == b.text
+
+
+def test_generation_prefix_does_not_widen_containment(client):
+    """The token is never joined into a path, so containment must be EXACTLY what the
+    un-prefixed route already gives. Asserted as parity rather than as a flat 404:
+    `../screen.js` legitimately 200s on BOTH, because the URL normalises to
+    /api/plugins/<id>/screen.js before routing ever happens — it never leaves the
+    plugin dir. Pinning an absolute expectation here would have encoded my guess
+    about the existing route instead of testing the thing that matters, which is
+    that /g/ changes nothing."""
+    c, _ = client
+    for bad in ("../screen.js", "../../etc/passwd", "..%2f..%2fetc%2fpasswd",
+                "..%5c..%5cwindows%5cwin.ini", "/etc/passwd"):
+        plain = c.get(f"/api/plugins/{PLUGIN_ID}/src/{bad}")
+        gen = c.get(f"/api/plugins/{PLUGIN_ID}/g/1/src/{bad}")
+        assert gen.status_code == plain.status_code, f"/g/ diverged on {bad!r}"
+        assert gen.content == plain.content, f"/g/ served different bytes for {bad!r}"
+        assert "root:" not in gen.text and "[extensions]" not in gen.text
+
+    # and the real traversals are genuinely rejected, on both
+    for bad in ("../../etc/passwd", "..%2f..%2fetc%2fpasswd"):
+        assert c.get(f"/api/plugins/{PLUGIN_ID}/g/1/src/{bad}").status_code == 404
+
+
+def test_generation_prefix_404s_for_unknown_plugin(client):
+    c, _ = client
+    assert c.get("/api/plugins/nope/g/1/screen.js").status_code == 404
+    assert c.get("/api/plugins/nope/g/1/src/main.js").status_code == 404
+
+
+def test_generation_prefix_serves_ASSETS_too(client):
+    """Codex [P2] on the first cut of this fix, and it was right.
+
+    The path token shifts the BASE URL, so everything a module resolves relatively moves
+    with it — not just imports. `new URL('../assets/worklet.js', import.meta.url)` from
+    /api/plugins/<id>/g/1/src/main.js resolves to /api/plugins/<id>/g/1/assets/worklet.js.
+    Mirroring only screen.js and src/ would have fixed imports and 404'd every asset,
+    worklet and wasm file the graph reaches. Hence a path REWRITE, so every plugin route
+    — present and future — works under the prefix."""
+    c, _ = client
+    plain = c.get(f"/api/plugins/{PLUGIN_ID}/assets/worklet.js")
+    gen = c.get(f"/api/plugins/{PLUGIN_ID}/g/1/assets/worklet.js")
+    assert plain.status_code == 200
+    assert gen.status_code == 200, "an asset reached relatively from a reloaded module graph 404'd"
+    assert gen.content == plain.content
+
+
+def test_generation_prefix_covers_every_plugin_route(client):
+    """The rewrite is generic, so this holds for routes nobody thought about — which is
+    the point. Any plugin route added later works under /g/ with no extra wiring."""
+    c, _ = client
+    for route in ("screen.js", "src/main.js", "src/util/x.js", "src/theme.css",
+                  "assets/worklet.js", "settings.html"):
+        plain = c.get(f"/api/plugins/{PLUGIN_ID}/{route}")
+        gen = c.get(f"/api/plugins/{PLUGIN_ID}/g/42/{route}")
+        assert gen.status_code == plain.status_code, f"/g/ diverged on {route}"
+        assert gen.content == plain.content, f"/g/ served different bytes for {route}"
+
+
+def test_generation_prefix_handles_non_ascii_filenames(client):
+    """Codex [P3] on the second cut. A plugin file named e.g. src/工具.js is perfectly
+    valid, and the middleware must not 500 on it — which an eager
+    raw_path.encode("latin-1") did, making the prefixed route LESS capable than the
+    plain one. raw_path is informational; Starlette routes on scope["path"]."""
+    c, tmp = client
+    (tmp / "src" / "工具.js").write_text("export const t = 1;\n")
+    plain = c.get(f"/api/plugins/{PLUGIN_ID}/src/工具.js")
+    gen = c.get(f"/api/plugins/{PLUGIN_ID}/g/3/src/工具.js")
+    assert plain.status_code == 200
+    assert gen.status_code == 200, "non-ASCII module path 500'd or 404'd under /g/"
+    assert gen.content == plain.content

tests/test_plugins.py

@@ -46,56 +46,6 @@ def capture_logger(caplog, logger_name, level=logging.WARNING):
         logger.propagate = orig_propagate
 
 
-# Bare module names that this test module pre-populates into
-# sys.modules to simulate the bare-import path. Saved/restored by
-# the reset_plugin_state fixture so they don't leak to other test
-# files. Codex / Copilot review on PR for feedBack#33.
-_BARE_NAMES_USED = ("util", "extractor")
-
-
-@pytest.fixture()
-def reset_plugin_state(monkeypatch):
-    """Clear loader module-level state and restore on teardown.
-
-    Saves and restores:
-      * `plugins.LOADED_PLUGINS`
-      * any `plugin_*` keys we add to `sys.modules`
-      * the bare names this module simulates (`util`, `extractor`)
-      * `sys.path` — `plugins.load_plugins()` mutates it
-    Also unsets `FEEDBACK_PLUGINS_DIR` for the test's duration
-    (via monkeypatch) so a CI env that pre-sets it can't leak
-    real user plugins into a tmp_path-driven test. Per-module
-    locks are owned by the standard import system
-    (`importlib._bootstrap._module_locks`) and are not our
-    responsibility to reset.
-    """
-    monkeypatch.delenv("FEEDBACK_PLUGINS_DIR", raising=False)
-    plugins = importlib.import_module("plugins")
-    saved_loaded = list(plugins.LOADED_PLUGINS)
-    saved_pending = dict(plugins.PENDING_PLUGINS)
-    saved_modules = {k: v for k, v in sys.modules.items() if k.startswith("plugin_")}
-    saved_bare = {k: sys.modules[k] for k in _BARE_NAMES_USED if k in sys.modules}
-    saved_path = list(sys.path)
-    plugins.LOADED_PLUGINS.clear()
-    plugins.PENDING_PLUGINS.clear()
-    for k in list(sys.modules):
-        if k.startswith("plugin_") or k in _BARE_NAMES_USED:
-            del sys.modules[k]
-    try:
-        yield plugins
-    finally:
-        plugins.LOADED_PLUGINS.clear()
-        plugins.LOADED_PLUGINS.extend(saved_loaded)
-        plugins.PENDING_PLUGINS.clear()
-        plugins.PENDING_PLUGINS.update(saved_pending)
-        for k in list(sys.modules):
-            if k.startswith("plugin_") or k in _BARE_NAMES_USED:
-                del sys.modules[k]
-        sys.modules.update(saved_modules)
-        sys.modules.update(saved_bare)
-        sys.path[:] = saved_path
-
-
 def _make_plugin(plugin_root, plugin_id, *, sibling_files=None, routes_body=None):
     """Create a minimal plugin directory under `plugin_root`.
 

tests/test_progression_api.py

@@ -5,6 +5,7 @@ import importlib
 import json
 import sys
 
+import builtin_content
 import pytest
 from fastapi.testclient import TestClient
 
@@ -192,7 +193,7 @@ def test_low_accuracy_play_does_not_complete_gated_challenge(client):
 
 
 def test_diagnostic_at_100_completes_calibration(client, server):
-    diag = server._builtin_diagnostic_filename()
+    diag = builtin_content.builtin_diagnostic_filename()
     # A near-miss leaves calibration pending.
     _scored_play(client, filename=diag, accuracy=0.97, score=500)
     assert client.get("/api/progression").json()["onboarding"]["calibration_status"] == "pending"
@@ -207,7 +208,7 @@ def test_diagnostic_play_does_not_feed_challenges_or_quests(client, server):
     # The calibration run is a perfect guitar play — it must yield rank 1
     # EXACTLY, advancing neither the guitar path nor the daily song quest.
     client.post("/api/progression/paths", json={"add": ["guitar"]})
-    r = _scored_play(client, filename=server._builtin_diagnostic_filename(),
+    r = _scored_play(client, filename=builtin_content.builtin_diagnostic_filename(),
                      accuracy=1.0, score=500)
     summary = r.json()["progression"]
     assert summary["calibration_completed"] is True
@@ -228,7 +229,7 @@ def test_pathless_diagnostic_run_still_completes_calibration(client, server):
     run is an earned achievement and must count even before any path is
     selected (e.g. a pre-progression profile playing the diagnostic as a
     hardware test) — yielding a valid pathless rank-1 state."""
-    _scored_play(client, filename=server._builtin_diagnostic_filename(),
+    _scored_play(client, filename=builtin_content.builtin_diagnostic_filename(),
                  accuracy=1.0, score=500)
     data = client.get("/api/progression").json()
     assert data["onboarding"]["calibration_status"] == "completed"
@@ -240,7 +241,7 @@ def test_diagnostic_upgrades_skipped_without_rank_change(client, server):
     client.post("/api/progression/paths", json={"add": ["guitar"]})
     r = client.post("/api/progression/onboarding", json={"action": "skip"})
     assert r.json()["onboarding"]["calibration_status"] == "skipped"
-    _scored_play(client, filename=server._builtin_diagnostic_filename(), accuracy=1.0, score=500)
+    _scored_play(client, filename=builtin_content.builtin_diagnostic_filename(), accuracy=1.0, score=500)
     data = client.get("/api/progression").json()
     assert data["onboarding"]["calibration_status"] == "completed"
     assert data["mastery_rank"] == 1