refactor(server): extract the enrichment route handlers into routers/enrichment.py (R3)

The 14 /api/enrichment/* routes — status, kick/cancel, per-song state, the
Match-Review queue (accept/reject/pick/search/rematch/refresh/states), and the
AcoustID fingerprint identify endpoints — plus the route-exclusive candidate
sanitizer move to lib/routers/enrichment.py. Bodies verbatim except @app->@router
and the seam reads (meta_db->appstate.meta_db, CONFIG_DIR->appstate.config_dir).
The enrichment engine (transport, matcher, worker, upload caps) already lives in
lib/enrichment.py from the earlier subsystem move and is reached as enrichment.X.
No new seams.

server.py: 2,925 -> 2,638 (-287).

Verified: pyflakes clean; route set identical (143); full pytest 2396 passed (the
enrichment route + Match-Review + identify cases, which fake the network on the
enrichment module). eslint 0.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

CHANGELOG.md

@@ -27,7 +27,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added
 - **Perf harness now measures 2D-highway frame time (R3c gate).** `scripts/perf-baseline.mjs` gains a `--song` mode that reports per-frame draw-cost p50/p95/p99 (draw-tagged via `highway.addDrawHook`), the metric that gates the `highway.js` split. Maintainer/CI-only; baseline recorded in `docs/perf-baseline.md`.
-- **`routers/` — extracting `server.py`'s route layer, cheapest-first (R3).** Each PR moves a cohesive route group into a `fastapi.APIRouter` under `lib/routers/`, mounted with `app.include_router(...)` at its original site (FastAPI matches in registration order; the full route table stays byte-identical). Bodies are verbatim — only the decorator receiver (`@app` → `@router`) and singleton reads (`meta_db` → `appstate.meta_db`, resolved at call time) change. So far: `audio_effects` (5), `artist_aliases` (5), `loops` (3), `playlists` (12 + covers), `ws_highway` (the 902-line highway chart WebSocket), `chart` (split/unsplit/work/fileinfo — unblocked by the DLC-path substrate), `library_extras`, `wanted`, `shop`, `progression`, `profile`, `stats` (the `/api/stats/{path}` catch-all stays registered last so it can't shadow `/recent` `/best` `/top`), `version` (`/api/version`; VERSION-file lookup adjusted for the router subdir depth), `art` (the `/api/song/{f}/art*` serve/cover-search/candidates/upload/url + `/api/art/{f}/override` routes; the shared `_song_pack_art_exists`/`_art_override_paths`/`_art_safe_name` helpers stay in `server.py` for the song/delete routes and are reached through the `appstate` seam, the CAA/release transport as `enrichment.X`), and `settings` (`GET`/`POST /api/settings`, `/reset`, and the two-phase atomic export/import bundle `/api/settings/export|import`; the shared `_default_settings` builder stays in `server.py` and is reached through the `appstate` seam), and `song` (upload/delete + the metadata write-back, user-meta, overrides, gap-fill, and per-song info routes; the scan/ingest helpers stay in `server.py` and are reached through new `appstate` seams — `kick_scan`, `invalidate_song_caches`, `stat_for_cache`, and a `scan_status()` getter — the `get_song_info` catch-all mounts after the art routes so it can't shadow them), and `diagnostics` (`/api/diagnostics/export|preview|hardware`; the plugins-root lookup adjusted for the router subdir depth, `_running_version` reached through the `appstate` seam, pure payload-cap helpers re-exported for the `server._diag_*` tests), and `tunings` (`/api/tunings`; the pure `config.json` reader moved to `lib/appconfig.py`, the tuning-provider registry read through the `appstate` seam so plugin-contributed tunings still merge). The DLC library-path resolution (`_get_dlc_dir`, pure `_resolve_dlc_path`) moved to `lib/dlc_paths.py`, reading paths through the seam; `config_dir`/`dlc_dir`/`dlc_dir_env` now ride the `appstate` seam (env-derived, so the pop-and-reimport fixtures reconfigure it for free), and the shared request-field sanitizer `_clean_str` moved to `lib/reqfields.py`. The next cut is picked by a dependency-closure scan that ranks groups by how many `monkeypatch.setattr(server, …)` targets they'd drag along.
+- **`routers/` — extracting `server.py`'s route layer, cheapest-first (R3).** Each PR moves a cohesive route group into a `fastapi.APIRouter` under `lib/routers/`, mounted with `app.include_router(...)` at its original site (FastAPI matches in registration order; the full route table stays byte-identical). Bodies are verbatim — only the decorator receiver (`@app` → `@router`) and singleton reads (`meta_db` → `appstate.meta_db`, resolved at call time) change. So far: `audio_effects` (5), `artist_aliases` (5), `loops` (3), `playlists` (12 + covers), `ws_highway` (the 902-line highway chart WebSocket), `chart` (split/unsplit/work/fileinfo — unblocked by the DLC-path substrate), `library_extras`, `wanted`, `shop`, `progression`, `profile`, `stats` (the `/api/stats/{path}` catch-all stays registered last so it can't shadow `/recent` `/best` `/top`), `version` (`/api/version`; VERSION-file lookup adjusted for the router subdir depth), `art` (the `/api/song/{f}/art*` serve/cover-search/candidates/upload/url + `/api/art/{f}/override` routes; the shared `_song_pack_art_exists`/`_art_override_paths`/`_art_safe_name` helpers stay in `server.py` for the song/delete routes and are reached through the `appstate` seam, the CAA/release transport as `enrichment.X`), and `settings` (`GET`/`POST /api/settings`, `/reset`, and the two-phase atomic export/import bundle `/api/settings/export|import`; the shared `_default_settings` builder stays in `server.py` and is reached through the `appstate` seam), and `song` (upload/delete + the metadata write-back, user-meta, overrides, gap-fill, and per-song info routes; the scan/ingest helpers stay in `server.py` and are reached through new `appstate` seams — `kick_scan`, `invalidate_song_caches`, `stat_for_cache`, and a `scan_status()` getter — the `get_song_info` catch-all mounts after the art routes so it can't shadow them), and `library` + collections (the provider list/art/sync endpoints, the library query surface, and collection CRUD → `lib/routers/library.py`; the `LibraryProviderRegistry`/`LocalLibraryProvider`/`SmartCollectionProvider` classes + shared query/collection helpers move to `lib/library_registry.py`, and the registry instance + local provider ride the `appstate` seam — server.py still constructs the singleton and exposes `register_library_provider`/`unregister_library_provider` to plugins via `plugin_context` unchanged), and the `enrichment` route handlers (`/api/enrichment/*`: status, kick/cancel, per-song state, the Match-Review queue, and AcoustID identify → `lib/routers/enrichment.py`; the engine already lives in `lib/enrichment.py` and is reached as `enrichment.X`), and `diagnostics` (`/api/diagnostics/export|preview|hardware`; the plugins-root lookup adjusted for the router subdir depth, `_running_version` reached through the `appstate` seam, pure payload-cap helpers re-exported for the `server._diag_*` tests), and `tunings` (`/api/tunings`; the pure `config.json` reader moved to `lib/appconfig.py`, the tuning-provider registry read through the `appstate` seam so plugin-contributed tunings still merge). The DLC library-path resolution (`_get_dlc_dir`, pure `_resolve_dlc_path`) moved to `lib/dlc_paths.py`, reading paths through the seam; `config_dir`/`dlc_dir`/`dlc_dir_env` now ride the `appstate` seam (env-derived, so the pop-and-reimport fixtures reconfigure it for free), and the shared request-field sanitizer `_clean_str` moved to `lib/reqfields.py`. The next cut is picked by a dependency-closure scan that ranks groups by how many `monkeypatch.setattr(server, …)` targets they'd drag along.
 - **`routers/` — the first extracted route module (R3).** The five audio-effects mapping
   endpoints move out of `server.py` into `lib/routers/audio_effects.py` as a
   `fastapi.APIRouter`, mounted with `app.include_router(...)` **at the point in the file

docs/size-exemptions.md

@@ -55,8 +55,8 @@ without a *signed* exemption" is unenforceable.
 ## Planned, NOT exempt (owned by split plans — listed so nothing falls between states)
 
 core `static/app.js` (11,852) · `static/highway.js` (4,168, whole file) · `server.py`
-(3,692 — was 14,037; ratcheted by the R3 `MetadataDB` + `AudioEffectsMappingDB`
-extractions and eighteen `routers/` modules (album-art in `lib/routers/art.py`, the settings + export/import bundle in `lib/routers/settings.py`); the ~930-line metadata-enrichment subsystem — MB/CAA/AcoustID transport, matcher, background worker — now lives in `lib/enrichment.py`) ·
+(2,638 — was 14,037; ratcheted by the R3 `MetadataDB` + `AudioEffectsMappingDB`
+extractions and twenty `routers/` modules, plus lib/library_registry.py for the provider-registry classes (album-art in `lib/routers/art.py`, the settings + export/import bundle in `lib/routers/settings.py`); the ~930-line metadata-enrichment subsystem — MB/CAA/AcoustID transport, matcher, background worker — now lives in `lib/enrichment.py`) ·
 `lib/metadata_db.py` (4,373 — new in R3; the `MetadataDB` class alone is 4,018 lines
 and is a monolith in its own right, to be split per-table once the router train
 lands) · `static/v3/songs.js` (4,134) · `static/capabilities/audio-session.js`

lib/appstate.py

@@ -65,6 +65,12 @@ audio_effect_mappings = None
 # stable object mutated in place via register()/unregister() — injected here by
 # reference so routers read the same registry plugins populate.
 tuning_providers = None
+# The library-provider registry instance + the local provider, constructed in
+# server.py (LocalLibraryProvider needs meta_db) and injected by reference. The
+# classes live in lib/library_registry.py; plugins register their own providers
+# through the registry via plugin_context.
+library_providers = None
+local_library_provider = None
 
 # Config paths. server.py derives these from the environment (fresh on every
 # import, so the ~49 pop-and-reimport fixtures keep working) and injects them
@@ -113,6 +119,7 @@ scan_status = None
 
 _SLOTS = frozenset({
     "meta_db", "audio_effect_mappings", "tuning_providers",
+    "library_providers", "local_library_provider",
     "config_dir", "dlc_dir", "dlc_dir_env",
     "static_dir", "sloppak_cache_dir", "audio_cache_dir",
     "get_progression_content", "builtin_diagnostic_filename",

lib/library_registry.py

@@ -0,0 +1,417 @@
+"""The library-provider registry — the plugin extension point for song sources.
+
+`LocalLibraryProvider` wraps the local `MetadataDB`; third-party plugins register
+their own providers (duck-typed: any object with the advertised methods) through
+`LibraryProviderRegistry`, and smart collections are surfaced as
+`SmartCollectionProvider`s over the local one. server.py constructs the singleton
+(`library_providers`), injects it + the local provider into appstate, and exposes
+`register_library_provider`/`unregister_library_provider` to plugins via
+plugin_context (with per-plugin ownership scoping in plugins/__init__.py).
+
+Moved verbatim out of server.py (R3). The shared query/collection helpers live
+here too so routers/library.py can import them without reaching into server.
+"""
+
+import re
+import threading
+from typing import ClassVar
+
+import appstate
+from metadata_db import MetadataDB, _tuning_group_key_sql
+from routers import art as art_router
+
+import logging
+log = logging.getLogger("feedBack.server")
+
+def _safe_art_redirect_url(url: str) -> str | None:
+    """Return the URL if it is safe to redirect to (http/https only), else None."""
+    from urllib.parse import urlparse
+    if not url or not isinstance(url, str):
+        return None
+    try:
+        parsed = urlparse(url)
+        if parsed.scheme.lower() not in ("http", "https"):
+            return None
+        if not parsed.hostname:
+            return None
+        return url
+    except Exception:
+        return None
+
+
+_TUNING_GROUP_KEY_SQL = _tuning_group_key_sql("songs")
+
+
+class LocalLibraryProvider:
+    id = "local"
+    label = "My Library"
+    kind = "local"
+    capabilities = (
+        "library.read",
+        "art.read",
+        "song.play",
+        "favorite.write",
+        "metadata.write",
+    )
+
+    def __init__(self, db: MetadataDB):
+        self._db = db
+
+    def query_page(self, **kwargs) -> tuple[list[dict], int]:
+        return self._db.query_page(**kwargs)
+
+    def query_artists(self, **kwargs) -> tuple[list[dict], int]:
+        return self._db.query_artists(**kwargs)
+
+    def query_albums(self, **kwargs) -> tuple[list[dict], int]:
+        return self._db.query_albums(**kwargs)
+
+    def query_stats(self, **kwargs) -> dict:
+        return self._db.query_stats(**kwargs)
+
+    def tuning_names(self) -> dict:
+        # Group custom tunings on their raw offsets so distinct ones stay
+        # distinct (tuning_name collapses them all to "Custom Tuning"); named
+        # tunings keep grouping by name (stable across the rescan boundary, no
+        # offsets/name split). `key` is the value the client sends back as the
+        # filter selector — equal to the name for named tunings, the offsets
+        # string for customs; offsets also feed the client's custom-pill label.
+        with self._db._lock:
+            rows = self._db.conn.execute(
+                f"SELECT tuning_name, {_TUNING_GROUP_KEY_SQL} AS gkey, "
+                "MIN(tuning_sort_key), COUNT(*), MIN(tuning_offsets) "
+                "FROM songs WHERE title != '' AND COALESCE(tuning_name, '') != '' "
+                "GROUP BY gkey COLLATE NOCASE "
+                "ORDER BY ABS(COALESCE(MIN(tuning_sort_key), 0)), "
+                "COALESCE(MIN(tuning_sort_key), 0) ASC, "
+                "tuning_name COLLATE NOCASE"
+            ).fetchall()
+        return {
+            "tunings": [
+                {"name": name, "key": gkey, "offsets": offs or "",
+                 "sort_key": int(sk or 0), "count": count}
+                for name, gkey, sk, count, offs in rows
+            ],
+        }
+
+    async def get_art(self, song_id: str):
+        return await art_router.get_song_art(song_id)
+
+
+class LibraryProviderRegistry:
+    # Methods required per declared capability — only validated when the
+    # provider advertises the corresponding capability so action-only providers
+    # (e.g. art.read + song.sync without library.read) don't need to implement
+    # unused stubs.
+    _CAPABILITY_METHODS: ClassVar[dict[str, tuple[str, ...]]] = {
+        "library.read": ("query_page", "query_artists", "query_stats", "tuning_names"),
+        "art.read": ("get_art",),
+        "song.sync": ("sync_song",),
+    }
+    _ID_RE: ClassVar[re.Pattern[str]] = re.compile(r"^[A-Za-z0-9][A-Za-z0-9_.:-]{0,127}$")
+
+    def __init__(self):
+        self._providers: dict[str, object] = {}
+        # Capabilities inferred at registration for legacy providers that omit
+        # the `capabilities` field.  Merged with provider_capabilities() so that
+        # runtime capability checks see the complete effective capability set.
+        self._inferred_caps: dict[str, set[str]] = {}
+        self._owner_plugin_ids: dict[str, str] = {}
+        self._lock = threading.RLock()
+
+    def register(self, provider: object, *, replace: bool = False, owner_plugin_id: str | None = None) -> object:
+        provider_id = self.provider_id(provider)
+        if not self._ID_RE.match(provider_id):
+            raise ValueError(
+                "library provider id must start with an alphanumeric character "
+                "and contain only letters, digits, _, ., :, or -"
+            )
+        if not self.provider_label(provider):
+            raise ValueError("library provider label must be a non-empty string")
+        # Use declared-only caps during validation — never include stale inferred
+        # caps from a previous provider registered under the same id (replace=True).
+        caps = self._declared_capabilities(provider)
+        # Backward compatibility: providers that predate explicit capability
+        # declarations may omit `capabilities` entirely. If the browse methods
+        # are all present, infer `library.read` so they still work unchanged.
+        # If capabilities are absent but the browse surface is also absent,
+        # raise a clear error rather than letting the provider register and
+        # then fail on every API call with a late 501.
+        inferred: set[str] = set()
+        if not caps:
+            browse_methods = self._CAPABILITY_METHODS["library.read"]
+            if all(callable(self.provider_method(provider, m)) for m in browse_methods):
+                # Legacy provider without explicit capabilities — infer library.read
+                # from the presence of all browse methods.  Store in _inferred_caps
+                # so that runtime capability checks see the full effective set.
+                inferred = {"library.read"}
+                caps = inferred
+            else:
+                raise TypeError(
+                    f"library provider {provider_id!r} must declare at least one capability "
+                    f"(or implement the {browse_methods!r} browse methods for backward compatibility)"
+                )
+        for cap, methods in self._CAPABILITY_METHODS.items():
+            if cap not in caps:
+                continue
+            for method_name in methods:
+                if not callable(self.provider_method(provider, method_name)):
+                    raise TypeError(f"library provider {provider_id!r} declares {cap!r} but is missing callable {method_name}()")
+        with self._lock:
+            if provider_id == "local" and provider_id in self._providers and self._providers[provider_id] is not provider:
+                raise ValueError("the local library provider cannot be replaced")
+            if provider_id in self._providers and not replace:
+                raise ValueError(f"library provider {provider_id!r} is already registered")
+            self._providers[provider_id] = provider
+            # owner_plugin_id is attribution that flows into the browser
+            # capability participant id. The scoped register_library_provider
+            # wrappers force it to the trusted loading plugin id, so the spoof
+            # vector is closed there. Here we only normalize: trim and require a
+            # non-empty string. We deliberately do NOT apply the provider-id
+            # grammar (_ID_RE) — plugin ids aren't constrained to it at load
+            # time, so that would silently drop attribution for valid plugins.
+            owner = owner_plugin_id.strip() if isinstance(owner_plugin_id, str) else ""
+            owner = owner or None
+            if owner:
+                self._owner_plugin_ids[provider_id] = owner
+            else:
+                self._owner_plugin_ids.pop(provider_id, None)
+            if inferred:
+                self._inferred_caps[provider_id] = inferred
+            else:
+                self._inferred_caps.pop(provider_id, None)
+        return provider
+
+    def unregister(self, provider_id: str) -> bool:
+        if provider_id == "local":
+            raise ValueError("the local library provider cannot be unregistered")
+        with self._lock:
+            self._inferred_caps.pop(provider_id, None)
+            self._owner_plugin_ids.pop(provider_id, None)
+            return self._providers.pop(provider_id, None) is not None
+
+    def get(self, provider_id: str = "local") -> object | None:
+        with self._lock:
+            return self._providers.get(provider_id or "local")
+
+    def list(self) -> list[dict]:
+        with self._lock:
+            providers = list(self._providers.values())
+        return [self.describe(provider) for provider in providers]
+
+    def describe(self, provider: object) -> dict:
+        provider_id = self.provider_id(provider)
+        with self._lock:
+            owner_plugin_id = self._owner_plugin_ids.get(provider_id)
+        return {
+            "id": provider_id,
+            "label": self.provider_label(provider),
+            "kind": self.provider_field(provider, "kind", "local" if provider_id == "local" else "remote"),
+            "capabilities": sorted(self.provider_capabilities(provider)),
+            "owner_plugin_id": owner_plugin_id,
+            "default": provider_id == "local",
+        }
+
+    def provider_field(self, provider: object, name: str, default=None):
+        if isinstance(provider, dict):
+            return provider.get(name, default)
+        return getattr(provider, name, default)
+
+    def provider_id(self, provider: object) -> str:
+        provider_id = self.provider_field(provider, "id", "")
+        if not isinstance(provider_id, str) or not provider_id:
+            raise ValueError("library provider id must be a non-empty string")
+        return provider_id
+
+    def provider_label(self, provider: object) -> str:
+        label = self.provider_field(provider, "label", self.provider_field(provider, "name", ""))
+        if not isinstance(label, str):
+            return ""
+        return label.strip()
+
+    def _declared_capabilities(self, provider: object) -> set[str]:
+        """Return only the capabilities explicitly declared on the provider object."""
+        raw = self.provider_field(provider, "capabilities", ())
+        if raw is None:
+            raw = ()
+        if isinstance(raw, str):
+            raw = (raw,) if raw else ()
+        return {str(cap) for cap in raw if cap}
+
+    def provider_capabilities(self, provider: object) -> set[str]:
+        # Guard against a common plugin authoring mistake: passing a single string
+        # instead of a list/tuple. Iterating a string produces individual characters,
+        # none of which would match a valid capability name.
+        declared = self._declared_capabilities(provider)
+        # Merge with any capabilities inferred at registration time for legacy
+        # providers that omit the `capabilities` field but implement browse methods.
+        provider_id = self.provider_id(provider)
+        with self._lock:
+            inferred = self._inferred_caps.get(provider_id, set())
+        return declared | inferred
+
+    def provider_method(self, provider: object, name: str):
+        if isinstance(provider, dict):
+            return provider.get(name)
+        return getattr(provider, name, None)
+
+
+# Keys `_library_filter_args` (and a smart collection's stored `rules`) accept.
+_LIBRARY_FILTER_PARAM_KEYS = frozenset((
+    "q", "favorites", "format", "artist", "album",
+    "arrangements_has", "arrangements_lacks", "stems_has", "stems_lacks",
+    "has_lyrics", "tunings",
+))
+
+
+# Rules mirror the raw /api/library query params (so the provider can feed them
+# straight through `_library_filter_args`, and the frontend can build a rule from
+# the same query string it already constructs). Multi-value filters are CSV
+# strings; `favorites` is 0/1; the rest are plain strings.
+_RULE_CSV_KEYS = frozenset((
+    "tunings", "arrangements_has", "arrangements_lacks", "stems_has", "stems_lacks",
+))
+
+
+_RULE_STR_KEYS = frozenset(("q", "format", "artist", "album", "has_lyrics", "sort"))
+
+
+def _sanitize_collection_rules(raw) -> dict:
+    """Normalize rules to the raw query-param format, keeping only known keys. A
+    list for a multi-value filter is joined to CSV; `favorites` becomes 0/1.
+    Unknown keys are dropped so a rule survives a filter-vocab change rather than
+    500-ing. Applied at API ingress AND when a provider loads a persisted row, so
+    a hand-edited / imported bad value (e.g. an int where a string is expected,
+    or a list for `sort`) can never crash a query."""
+    if not isinstance(raw, dict):
+        return {}
+    out: dict = {}
+    for k, v in raw.items():
+        if k in _RULE_CSV_KEYS:
+            if isinstance(v, list):
+                vals = [str(x) for x in v if isinstance(x, (str, int)) and not isinstance(x, bool)]
+            elif isinstance(v, str):
+                vals = [s for s in (p.strip() for p in v.split(",")) if s]
+            else:
+                continue
+            if vals:
+                out[k] = ",".join(vals)
+        elif k == "favorites":
+            if v:
+                out[k] = 1
+        elif k in _RULE_STR_KEYS:
+            if isinstance(v, (str, int)) and not isinstance(v, bool):
+                s = str(v).strip()
+                if s:
+                    out[k] = s
+    return out
+
+
+class SmartCollectionProvider:
+    """A saved library filter, surfaced as a source (#636 item 2). Browse/stats
+    delegate to the local DB with the collection's stored `rules` applied — so
+    selecting it in the v3 source picker shows exactly that filtered slice with
+    the whole Songs UI (paging, stats, A–Z rail, art) for free. P1: the rules
+    ARE the query (live in-collection search is a P2 nicety). The matched songs
+    are local rows, so `kind="local"` keeps the client's play/art paths on the
+    local (not remote-sync) branch and art delegates straight through."""
+    kind = "local"
+    capabilities = ("library.read", "art.read")
+
+    def __init__(self, collection: dict, local: "LocalLibraryProvider"):
+        self._local = local
+        self.update(collection)
+
+    def update(self, collection: dict) -> None:
+        self.id = f"collection:{collection['id']}"
+        self.collection_id = collection["id"]
+        self.label = collection.get("name") or "Collection"
+        # Re-sanitize on load: persisted JSON may predate the current vocab or
+        # have been hand-edited; never let a bad value reach a query.
+        self._rules = _sanitize_collection_rules(collection.get("rules") or {})
+
+    def _filter_kwargs(self) -> dict:
+        return _library_filter_args(**{k: v for k, v in self._rules.items()
+                                       if k in _LIBRARY_FILTER_PARAM_KEYS})
+
+    def _sort(self, fallback: str) -> str:
+        # A collection may pin its own sort (e.g. "recently added"); query_page
+        # falls back safely for an unknown value, so no validation needed here.
+        return self._rules.get("sort") or fallback
+
+    def query_page(self, *, page=0, size=24, sort="artist", direction="asc",
+                   naming_mode="legacy", **_ignore):
+        return self._local._db.query_page(
+            page=page, size=size, sort=self._sort(sort), direction=direction,
+            naming_mode=naming_mode, **self._filter_kwargs())
+
+    def query_artists(self, *, letter="", page=0, size=50, naming_mode="legacy", **_ignore):
+        return self._local._db.query_artists(
+            letter=letter, page=page, size=size, naming_mode=naming_mode,
+            **self._filter_kwargs())
+
+    def query_albums(self, *, page=0, size=120, naming_mode="legacy", **_ignore):
+        return self._local._db.query_albums(
+            page=page, size=size, naming_mode=naming_mode, **self._filter_kwargs())
+
+    def query_stats(self, *, sort="artist", want_sort_letters=False,
+                    naming_mode="legacy", **_ignore):
+        return self._local._db.query_stats(
+            sort=self._sort(sort), want_sort_letters=want_sort_letters,
+            naming_mode=naming_mode, **self._filter_kwargs())
+
+    def tuning_names(self):
+        return self._local.tuning_names()
+
+    async def get_art(self, song_id: str):
+        return await self._local.get_art(song_id)
+
+
+def _split_csv(raw: str) -> list[str]:
+    """Parse a comma-separated query-string list. Empty / whitespace-only
+    entries are dropped so `arrangements_has=` (no value) and
+    `arrangements_has=,` both mean 'no filter'."""
+    if not raw:
+        return []
+    return [s.strip() for s in raw.split(",") if s.strip()]
+
+
+def _parse_has_lyrics(raw: str) -> int | None:
+    """Tri-state parse for has_lyrics. `1` → require, `0` → exclude,
+    anything else (including empty) → no filter."""
+    if raw == "1":
+        return 1
+    if raw == "0":
+        return 0
+    return None
+
+
+def _library_filter_args(q: str = "", favorites: int = 0, format: str = "",
+                         artist: str = "", album: str = "",
+                         arrangements_has: str = "", arrangements_lacks: str = "",
+                         stems_has: str = "", stems_lacks: str = "",
+                         has_lyrics: str = "", tunings: str = "") -> dict:
+    fmt = format if format in ("archive", "sloppak", "loose") else ""
+    return {
+        "q": q,
+        "favorites_only": bool(favorites),
+        "format_filter": fmt,
+        "artist_filter": (artist or "").strip(),
+        "album_filter": (album or "").strip(),
+        "arrangements_has": _split_csv(arrangements_has),
+        "arrangements_lacks": _split_csv(arrangements_lacks),
+        "stems_has": _split_csv(stems_has),
+        "stems_lacks": _split_csv(stems_lacks),
+        "has_lyrics": _parse_has_lyrics(has_lyrics),
+        "tunings": _split_csv(tunings),
+    }
+
+
+def _sync_collection_provider(collection: dict) -> None:
+    """Register (or replace) the provider for one collection."""
+    appstate.library_providers.register(
+        SmartCollectionProvider(collection, appstate.local_library_provider), replace=True)
+
+
+def _unregister_collection_provider(pid: int) -> None:
+    appstate.library_providers.unregister(f"collection:{pid}")

lib/routers/enrichment.py

@@ -0,0 +1,346 @@
+"""Metadata-enrichment route handlers (/api/enrichment/*): status, kick/cancel,
+per-song state, the Match-Review queue (accept/reject/pick/search), and AcoustID
+fingerprint identify.
+
+Extracted verbatim from server.py (R3) except @app->@router and the seam reads
+(meta_db->appstate.meta_db, CONFIG_DIR->appstate.config_dir). The enrichment
+engine itself — transport, matcher, the background worker, and the upload caps —
+lives in lib/enrichment.py and is reached here as enrichment.X.
+"""
+
+import asyncio
+import os
+import shutil
+from pathlib import Path
+
+from fastapi import APIRouter, Body, HTTPException, Request, UploadFile
+from fastapi.responses import JSONResponse
+
+import appstate
+import enrichment
+import mb_match
+from appconfig import _load_config
+
+import logging
+log = logging.getLogger("feedBack.server")
+router = APIRouter()
+
+@router.get("/api/enrichment/status")
+def enrichment_status():
+    """Enrichment pipeline state: worker flags + row counts by match_state.
+    Ambient tool-state for the match-review UI (never a home-screen score —
+    design §11); also what tests poke."""
+    return {
+        "running": enrichment._enrich_status["running"],
+        "processed": enrichment._enrich_status["processed"],
+        "last_pass_at": enrichment._enrich_status["last_pass_at"],
+        "states": appstate.meta_db.enrichment_state_counts(),
+        "total_songs": appstate.meta_db.count(),
+        # Per-pass matching progress for the "Refresh Metadata" batch bar +
+        # per-tile badges (total = songs queued to match this pass, matched =
+        # done so far, current = the one being matched now).
+        "total": enrichment._enrich_status.get("total", 0),
+        "matched": enrichment._enrich_status.get("matched", 0),
+        "current": enrichment._enrich_status.get("current"),
+        "cancelling": enrichment._enrich_cancel.is_set(),
+    }
+
+
+@router.get("/api/enrichment/song/{filename:path}")
+def api_enrichment_song(filename: str):
+    """Read-only per-song match provenance for the Details drawer (launch
+    polish): which canonical identity this chart matched and how. A tiny
+    projection of the cache row — no candidates, no cache paths."""
+    row = appstate.meta_db.get_enrichment(filename)
+    if not row:
+        raise HTTPException(status_code=404, detail="no enrichment row")
+    return {k: row.get(k) for k in
+            ("match_state", "canon_artist", "canon_title",
+             "match_source", "match_score")}
+
+
+@router.post("/api/enrichment/kick")
+def api_enrichment_kick():
+    """The Settings "Match now" button AND the library's "Refresh Metadata"
+    button: request an enrichment pass without waiting for a scan to complete.
+    Processes the songs that still need it (unscanned/changed + retriable
+    failures) — already-matched songs are left alone, so on a fully-matched
+    library this is a fast no-op. Single-flight + coalescing like every other
+    kick — spamming it queues at most one follow-up pass."""
+    return {"started": enrichment._kick_enrich()}
+
+
+@router.post("/api/enrichment/cancel")
+def api_enrichment_cancel():
+    """Stop button on the "Refresh Metadata" batch: signal the running pass to
+    halt after the current song (an in-flight ≤1/s lookup can't be interrupted,
+    but no new one is started) and drop any coalesced follow-up. A no-op when
+    nothing is running."""
+    was_running = enrichment._enrich_status["running"]
+    if was_running:
+        enrichment._enrich_cancel.set()
+    return {"ok": True, "was_running": was_running}
+
+
+@router.post("/api/enrichment/rematch")
+def api_enrichment_rematch(data: dict = Body(...)):
+    """The library "Refresh Metadata" button: force a fresh re-match of the
+    songs the grid is SHOWING (its visible/filtered window). Resets each to
+    `unscanned` so the next pass re-fetches it from scratch — EXCEPT user-pinned
+    `manual` rows, which are never auto-overwritten (apply_enrichment_match
+    guards that) — then kicks one pass. Scoped to the visible set on purpose:
+    fast (dozens of songs), visible (tiles animate), and it can't blow the whole
+    ≤1/s rate budget on a 1000-song library the way a full re-sweep would.
+    Returns the filenames actually queued so the UI badges exactly those."""
+    raw = (data or {}).get("filenames") or []
+    fns = [str(f) for f in raw if isinstance(f, str)][:500]
+    queued: list[str] = []
+    for fn in fns:
+        song = appstate.meta_db.enrichment_song_row(fn)
+        if not song:
+            continue
+        h = appstate.meta_db.enrichment_content_hash(
+            song["artist"], song["title"], song["album"], song["duration"])
+        # allow_manual_overwrite=False → a manual pin is left as-is (returns
+        # False), everything else resets to unscanned (returns True).
+        if appstate.meta_db.apply_enrichment_match(fn, h, "unscanned",
+                                          allow_manual_overwrite=False):
+            queued.append(fn)
+    started = enrichment._kick_enrich() if queued else False
+    return {"queued": queued, "count": len(queued), "started": started}
+
+
+@router.post("/api/enrichment/states")
+def api_enrichment_states(data: dict = Body(...)):
+    """Per-tile match states for the grid's VISIBLE window during a metadata
+    refresh: the client posts the filenames it is showing and gets back each
+    one's match_state (+ the song being matched right now, + whether a pass is
+    running), so a card can animate queued→working→result without a per-song
+    round-trip. Read-only — safe for demo visitors (no network, no mutation)."""
+    raw = (data or {}).get("filenames") or []
+    # Bound the batch: a visible grid window is dozens of cards; cap defensively.
+    fns = [str(f) for f in raw if isinstance(f, str)][:500]
+    return {
+        "states": appstate.meta_db.enrichment_states_for(fns),
+        "current": enrichment._enrich_status.get("current"),
+        "running": enrichment._enrich_status["running"],
+    }
+
+
+@router.post("/api/enrichment/refresh/{filename:path}")
+def api_enrichment_refresh(filename: str):
+    """The context menu's "Refresh metadata": reset THIS song's match to
+    unscanned (canonical values + candidates cleared, backoff zeroed) and
+    kick a pass so it re-matches immediately. An EXPLICIT user action, so it
+    may discard a manual pin — the automation never does, but the user
+    asking for a re-match is the one party who owns that pin."""
+    song = appstate.meta_db.enrichment_song_row(filename)
+    if not song:
+        raise HTTPException(status_code=404, detail="unknown song")
+    h = appstate.meta_db.enrichment_content_hash(
+        song["artist"], song["title"], song["album"], song["duration"])
+    appstate.meta_db.apply_enrichment_match(filename, h, "unscanned",
+                                   allow_manual_overwrite=True)
+    return {"ok": True, "started": enrichment._kick_enrich()}
+
+
+@router.get("/api/enrichment/review")
+def api_enrichment_review(limit: int = 200):
+    """The Match-Review queue: songs whose text match landed in the medium-
+    confidence review tier, each with its stored candidate list — the drawer
+    renders straight from this, no MusicBrainz round-trip. Ordered by the
+    user's enrich_review_order setting."""
+    limit = max(1, min(int(limit), 500))
+    cfg = _load_config(appstate.config_dir / "config.json") or {}
+    order = cfg.get("enrich_review_order", "missing_first")
+    return {
+        "songs": appstate.meta_db.enrichment_review_queue(limit=limit, order=order),
+        "total_review": appstate.meta_db.enrichment_state_counts().get("review", 0),
+    }
+
+
+@router.post("/api/enrichment/review/{filename:path}/accept")
+def api_enrichment_accept(filename: str, data: dict = Body(...)):
+    """Accept one of the stored review candidates: the row becomes a
+    user-pinned `manual` match (never auto-reset). Display-only, like every
+    enrichment write — nothing touches the pack file."""
+    recording_id = str((data or {}).get("recording_id") or "")
+    row = appstate.meta_db.get_enrichment(filename)
+    if not row or row["match_state"] != "review":
+        raise HTTPException(status_code=404, detail="no review row for this song")
+    cand = next((c for c in (row.get("candidates") or [])
+                 if c.get("recording_id") == recording_id), None)
+    if not cand:
+        raise HTTPException(status_code=404, detail="candidate not in the stored list")
+    if not appstate.meta_db.set_enrichment_manual(filename, cand, source="review"):
+        raise HTTPException(status_code=404, detail="unknown song")
+    return {"ok": True, "enrichment": appstate.meta_db.get_enrichment(filename)}
+
+
+@router.post("/api/enrichment/review/{filename:path}/reject")
+def api_enrichment_reject(filename: str):
+    """"None of these" — clears any canonical values and parks the row as
+    failed/rejected (never auto-retried; editing the song's metadata
+    re-queues it). Valid from `review` or `matched`, never from `manual`."""
+    if not appstate.meta_db.set_enrichment_rejected(filename):
+        raise HTTPException(status_code=404, detail="no rejectable match for this song")
+    return {"ok": True, "enrichment": appstate.meta_db.get_enrichment(filename)}
+
+
+# The candidate fields a manual pick is allowed to carry — the payload comes
+# from our own /api/enrichment/search proxy, but the route re-sanitizes so a
+# hand-rolled client can't stuff arbitrary keys/types into the cache row.
+_CAND_STR_FIELDS = ("recording_id", "title", "artist", "artist_id",
+                    "artist_sort", "release_id", "album", "year", "isrc")
+
+
+def _sanitize_candidate(raw: dict) -> dict | None:
+    if not isinstance(raw, dict):
+        return None
+    out = {k: str(raw.get(k) or "") for k in _CAND_STR_FIELDS}
+    if not out["recording_id"] or not out["title"]:
+        return None
+    genres = raw.get("genres") or []
+    out["genres"] = [str(g) for g in genres if isinstance(g, str)][:5] \
+        if isinstance(genres, list) else []
+    return out
+
+
+@router.post("/api/enrichment/review/{filename:path}/pick")
+def api_enrichment_pick(filename: str, data: dict = Body(...)):
+    """Fix-match / manual search-and-pick: pin a candidate the user found via
+    /api/enrichment/search (not limited to the stored review list — this is
+    the escape hatch for a wrong auto-match too). Sets `manual`, the
+    highest-authority state."""
+    cand = _sanitize_candidate((data or {}).get("candidate"))
+    if not cand:
+        raise HTTPException(status_code=400, detail="candidate needs recording_id + title")
+    if not appstate.meta_db.set_enrichment_manual(filename, cand, source="search"):
+        raise HTTPException(status_code=404, detail="unknown song")
+    return {"ok": True, "enrichment": appstate.meta_db.get_enrichment(filename)}
+
+
+@router.get("/api/enrichment/search")
+def api_enrichment_search(artist: str = "", title: str = "", limit: int = 8,
+                          filename: str = "", duration: float = 0.0):
+    """Manual-search proxy to MusicBrainz (throttled + identified like the
+    background matcher — a user typing in the drawer must not sidestep the
+    rate limit). `filename` optionally scores results against that song's
+    stored identity (year/duration corroboration) instead of just the typed
+    text. `duration` (seconds) lets a caller that HAS the audio but no library
+    row — e.g. the editor's create modal, which holds the master track — pass
+    its length so the studio take ranks above live/extended cuts. Sync route on
+    purpose: FastAPI runs it in the threadpool, so the throttle's sleep never
+    blocks the event loop."""
+    if not (artist.strip() or title.strip()):
+        raise HTTPException(status_code=400, detail="artist or title required")
+    limit = max(1, min(int(limit), 25))
+    try:
+        cands = enrichment._mb_search_recordings(artist, title, limit=limit)
+    except enrichment.EnrichTransportError as e:
+        return JSONResponse({"error": "musicbrainz unavailable", "detail": str(e)},
+                            status_code=503)
+    ref = None
+    if filename:
+        ref = appstate.meta_db.enrichment_song_row(filename)
+    if ref is None:
+        ref = {"artist": artist, "title": title}
+    # A caller-supplied duration corroborates the take even without a library row.
+    if duration and duration > 0 and not ref.get("duration"):
+        ref = dict(ref)
+        ref["duration"] = duration
+    # Alias-enrich so a non-Latin-primary artist (大橋純子) ranks by its
+    # romanized alias against the typed query ("Junko Ohashi") instead of
+    # sinking to the bottom with a 0 artist score.
+    try:
+        enrichment._alias_enrich(ref, cands)
+    except enrichment.EnrichTransportError:
+        pass   # aliases are a ranking nicety here; fall back to primary-name scoring
+    return {"candidates": mb_match.rank_candidates(ref, cands)}
+
+
+@router.post("/api/enrichment/identify")
+async def api_enrichment_identify(request: Request):
+    """Identify a song by AUDIO FINGERPRINT (AcoustID) rather than text — the
+    reliable way to get the EXACT recording/version (the studio take, not a live
+    bootleg or an extended cut). Upload the master audio; returns candidates in
+    the same shape as /search, so the review UI and the editor's Match popup can
+    render fingerprint hits identically. 412 `needs_setup` when the user hasn't
+    opted in / has no key (the UI nudges them to Settings); 503 when it's set up
+    but the fpcalc Chromaprint binary is missing or the network is off. Async so
+    the multipart is size-capped BEFORE spooling; the blocking fpcalc subprocess
+    + AcoustID HTTP run in the threadpool via run_in_executor."""
+    gate = enrichment._acoustid_gate()
+    if gate is not None:
+        return gate
+    # Pre-parse Content-Length guard — reject an oversized body before Starlette
+    # spools the multipart to temp disk (mirrors the song-upload endpoint). The
+    # per-part cap below is the authoritative limit; this is the fast up-front no.
+    cl = request.headers.get("content-length")
+    if cl is not None:
+        try:
+            cl_int = int(cl)
+        except ValueError:
+            return JSONResponse({"error": "Invalid Content-Length header"}, status_code=400)
+        if cl_int > enrichment._ACOUSTID_MAX_UPLOAD_BYTES + enrichment._MULTIPART_OVERHEAD_SLACK:
+            return JSONResponse({"error": "audio upload too large (256 MB max)"}, status_code=413)
+    try:
+        form = await request.form(max_part_size=enrichment._ACOUSTID_MAX_UPLOAD_BYTES)
+    except Exception:
+        return JSONResponse({"error": "audio upload too large (256 MB max)"}, status_code=413)
+    file = form.get("file")
+    if not isinstance(file, UploadFile):
+        raise HTTPException(status_code=400, detail="missing file upload")
+    import tempfile
+    ext = (Path(file.filename or "").suffix or ".bin").lower()
+    tmpdir = tempfile.mkdtemp(prefix="feedback_acoustid_")
+    tmp = os.path.join(tmpdir, "audio" + ext)
+    try:
+        total = 0
+        with open(tmp, "wb") as fh:
+            while True:
+                chunk = await file.read(1024 * 1024)
+                if not chunk:
+                    break
+                total += len(chunk)
+                if total > enrichment._ACOUSTID_MAX_UPLOAD_BYTES:
+                    return JSONResponse(
+                        {"error": "audio upload too large (256 MB max)"}, status_code=413)
+                fh.write(chunk)
+        if total == 0:
+            raise HTTPException(status_code=400, detail="empty upload")
+        # fpcalc subprocess + AcoustID HTTP are blocking — off the event loop.
+        cands = await asyncio.get_event_loop().run_in_executor(
+            None, enrichment._identify_by_fingerprint, tmp)
+    except enrichment.EnrichTransportError as e:
+        return JSONResponse({"error": "acoustid unavailable", "detail": str(e)},
+                            status_code=503)
+    finally:
+        shutil.rmtree(tmpdir, ignore_errors=True)
+    return {"candidates": cands}
+
+
+@router.post("/api/enrichment/identify/{filename:path}")
+def api_enrichment_identify_song(filename: str):
+    """Identify an EXISTING library song by AUDIO FINGERPRINT — the library-side
+    counterpart to /api/enrichment/identify (which takes an upload). Fingerprints
+    the song's own master audio on disk (the manual "Identify by audio" action in
+    the Fix-metadata / match-review flow). Same candidate shape as /search, so the
+    review UI renders fingerprint hits like text hits. Same 412/503 gating; 404
+    when the song has no full-mix audio to fingerprint."""
+    gate = enrichment._acoustid_gate()
+    if gate is not None:
+        return gate
+    audio = enrichment._song_audio_file(filename)
+    if not audio:
+        return JSONResponse(
+            {"error": "no audio",
+             "detail": "couldn't find this song's master audio to fingerprint "
+                       "(a stems-only pack has no full mix to identify)."},
+            status_code=404)
+    try:
+        cands = enrichment._identify_by_fingerprint(audio)
+    except enrichment.EnrichTransportError as e:
+        return JSONResponse({"error": "acoustid unavailable", "detail": str(e)},
+                            status_code=503)
+    return {"candidates": cands}

lib/routers/library.py

@@ -0,0 +1,485 @@
+"""Library + smart-collection routes: the provider list/art/sync endpoints, the
+library query surface (songs, albums, artists, stats, genres, tuning-names,
+practice-suggestions), and collection CRUD.
+
+Extracted verbatim from server.py (R3) except @app->@router and the seam reads:
+meta_db->appstate.meta_db, and the registry singletons ->
+appstate.library_providers / appstate.local_library_provider (constructed +
+owned by server.py; plugins register providers through plugin_context). The
+provider classes + shared query/collection helpers live in lib/library_registry.py.
+"""
+
+import inspect
+from pathlib import Path
+from typing import Any
+
+from fastapi import APIRouter, HTTPException
+from fastapi.responses import FileResponse, JSONResponse, RedirectResponse, Response
+from starlette.concurrency import run_in_threadpool
+
+import appstate
+from library_registry import (
+    _library_filter_args, _sanitize_collection_rules,
+    _safe_art_redirect_url, _split_csv, _sync_collection_provider,
+    _unregister_collection_provider,
+)
+from metadata_db import _effective_keyset_sort, next_library_cursor
+from reqfields import _clean_str
+
+import logging
+log = logging.getLogger("feedBack.server")
+router = APIRouter()
+
+
+
+
+
+def _get_library_provider(provider: str = "local") -> object:
+    library_provider = appstate.library_providers.get(provider or "local")
+    if library_provider is None:
+        raise HTTPException(status_code=404, detail=f"Unknown library provider: {provider}")
+    return library_provider
+
+
+def _require_library_provider_capability(provider: object, capability: str) -> None:
+    if capability in appstate.library_providers.provider_capabilities(provider):
+        return
+    provider_id = appstate.library_providers.provider_id(provider)
+    raise HTTPException(
+        status_code=501,
+        detail=f"Library provider {provider_id!r} does not declare capability {capability!r}",
+    )
+
+
+_OPTIONAL_NEW_PROVIDER_KWARGS = ("naming_mode", "sort", "want_sort_letters", "after",
+                                 "mastery", "match_states")
+
+
+def _filter_provider_kwargs(method: object, kwargs: dict) -> dict:
+    """Drop kwargs that the method's signature does not declare.
+
+    Provides backward-compat for third-party library providers whose
+    query_page/query_artists/query_stats methods were written before
+    naming_mode was added — calling them with the extra kwarg would
+    raise TypeError and return a 500 to the client.
+
+    When ``inspect.signature`` cannot introspect the method (rare: C
+    extensions / built-ins / exotic callables), fall back to stripping
+    only the kwargs we know were added later — older providers won't
+    accept them, anything else stays so the call still works.
+    """
+    try:
+        sig = inspect.signature(method)  # type: ignore[arg-type]
+        for p in sig.parameters.values():
+            if p.kind == inspect.Parameter.VAR_KEYWORD:
+                return kwargs  # method accepts **kwargs, pass everything
+        return {k: v for k, v in kwargs.items() if k in sig.parameters}
+    except (ValueError, TypeError):
+        return {k: v for k, v in kwargs.items() if k not in _OPTIONAL_NEW_PROVIDER_KWARGS}
+
+
+def _call_library_provider(provider: object, method_name: str, **kwargs) -> Any:
+    method = appstate.library_providers.provider_method(provider, method_name)
+    if not callable(method):
+        provider_id = appstate.library_providers.provider_id(provider)
+        raise HTTPException(
+            status_code=501,
+            detail=f"Library provider {provider_id!r} does not support {method_name}",
+        )
+    try:
+        return method(**_filter_provider_kwargs(method, kwargs))
+    except HTTPException:
+        raise
+    except Exception as exc:
+        provider_id = appstate.library_providers.provider_id(provider)
+        # A provider with an explicit kind="local" is treated as local even if
+        # its id is not "local" (e.g. a kind="local" plugin variant). Otherwise
+        # fall back to provider_id comparison so providers that omit `kind` are
+        # still wrapped correctly — the safe default for unknown providers is to
+        # surface an offline message rather than leaking raw exceptions.
+        provider_kind = str(appstate.library_providers.provider_field(provider, "kind", "") or "")
+        if provider_kind:
+            is_remote = provider_kind not in ("", "local")
+        else:
+            is_remote = provider_id != "local"
+        if is_remote:
+            detail = f"This source appears to be offline ({provider_id})."
+            message = str(exc).strip()
+            if message:
+                detail = f"{detail} {message}"
+            raise HTTPException(status_code=503, detail=detail) from exc
+        raise
+
+
+def _is_async_callable(obj: object) -> bool:
+    """Return True if obj is an async function or a callable object with an async __call__.
+
+    ``inspect.iscoroutinefunction`` only recognises bare coroutine functions; it returns
+    False for class instances whose ``__call__`` method is defined as ``async def``.
+    Checking both handles the common plugin pattern of wrapping an async method in a
+    callable object.
+    """
+    if inspect.iscoroutinefunction(obj):
+        return True
+    _call = getattr(obj, "__call__", None)
+    return _call is not None and inspect.iscoroutinefunction(_call)
+
+
+async def _call_library_provider_async(provider: object, method_name: str, **kwargs) -> Any:
+    method = appstate.library_providers.provider_method(provider, method_name)
+    if _is_async_callable(method):
+        # Async provider method — call directly on the event loop.
+        try:
+            return await method(**_filter_provider_kwargs(method, kwargs))
+        except HTTPException:
+            raise
+        except Exception as exc:
+            provider_id = appstate.library_providers.provider_id(provider)
+            provider_kind = str(appstate.library_providers.provider_field(provider, "kind", "") or "")
+            if provider_kind:
+                is_remote = provider_kind not in ("", "local")
+            else:
+                is_remote = provider_id != "local"
+            if is_remote:
+                detail = f"This source appears to be offline ({provider_id})."
+                message = str(exc).strip()
+                if message:
+                    detail = f"{detail} {message}"
+                raise HTTPException(status_code=503, detail=detail) from exc
+            raise
+    # Synchronous provider method — run in a threadpool so the event loop stays free.
+    return await run_in_threadpool(_call_library_provider, provider, method_name, **kwargs)
+
+
+def _library_art_response(result: Any) -> Response:
+    if result is None:
+        raise HTTPException(status_code=404, detail="Library provider returned no art")
+    if isinstance(result, Response):
+        return result
+    if isinstance(result, (bytes, bytearray, memoryview)):
+        return Response(content=bytes(result), media_type="image/png")
+    if isinstance(result, str):
+        safe_url = _safe_art_redirect_url(result)
+        if safe_url is not None:
+            return RedirectResponse(safe_url)
+        # If the string looks like a URL (contains a scheme separator) but
+        # didn't pass the http/https check, refuse it rather than treating
+        # it as a filesystem path — a provider returning ftp:// or file://
+        # should get a 400, not a 500 from FileResponse failing on a URL.
+        if "://" in result:
+            raise HTTPException(
+                status_code=400,
+                detail="Library provider returned an unsupported URL scheme for art",
+            )
+        if not Path(result).is_file():
+            raise HTTPException(status_code=404, detail="Library provider returned an unreadable art path")
+        return FileResponse(result)
+    if isinstance(result, Path):
+        if not result.is_file():
+            raise HTTPException(status_code=404, detail="Library provider returned an unreadable art path")
+        return FileResponse(str(result))
+    if isinstance(result, dict):
+        url = result.get("url") or result.get("art_url") or result.get("artUrl")
+        if isinstance(url, str) and url:
+            safe_url = _safe_art_redirect_url(url)
+            if safe_url is None:
+                raise HTTPException(status_code=400, detail="Library provider returned an unsafe art URL")
+            return RedirectResponse(safe_url)
+        path = result.get("path") or result.get("file")
+        if isinstance(path, (str, Path)):
+            media_type = result.get("media_type") or result.get("content_type")
+            if not Path(path).is_file():
+                raise HTTPException(status_code=404, detail="Library provider returned an unreadable art path")
+            return FileResponse(str(path), media_type=media_type)
+        content = result.get("content") or result.get("bytes")
+        if isinstance(content, (bytes, bytearray, memoryview)):
+            media_type = result.get("media_type") or result.get("content_type") or "image/png"
+            return Response(content=bytes(content), media_type=media_type)
+    raise HTTPException(status_code=500, detail="Library provider returned unsupported art data")
+
+
+@router.get("/api/library/providers")
+def list_library_providers():
+    """List registered library providers."""
+    return {"providers": appstate.library_providers.list()}
+
+
+@router.get("/api/library/providers/{provider_id}/songs/{song_id:path}/art")
+async def get_library_provider_song_art(provider_id: str, song_id: str):
+    """Return album art for a song owned by a library provider."""
+    library_provider = _get_library_provider(provider_id)
+    _require_library_provider_capability(library_provider, "art.read")
+    result = await _call_library_provider_async(library_provider, "get_art", song_id=song_id)
+    return _library_art_response(result)
+
+
+@router.post("/api/library/providers/{provider_id}/songs/{song_id:path}/sync")
+async def sync_library_provider_song(provider_id: str, song_id: str):
+    """Ask a provider to sync a remote song into the local library/cache."""
+    library_provider = _get_library_provider(provider_id)
+    _require_library_provider_capability(library_provider, "song.sync")
+    result = await _call_library_provider_async(library_provider, "sync_song", song_id=song_id)
+    if result is None:
+        return {"ok": True}
+    if isinstance(result, dict):
+        return result
+    return {"ok": True, "result": result}
+
+
+@router.get("/api/library")
+async def list_library(q: str = "", page: int = 0, size: int = 24, sort: str = "artist",
+                       dir: str = "asc", favorites: int = 0, format: str = "",
+                       artist: str = "", album: str = "",
+                       arrangements_has: str = "", arrangements_lacks: str = "",
+                       stems_has: str = "", stems_lacks: str = "",
+                       has_lyrics: str = "", tunings: str = "", provider: str = "local",
+                       mastery: str = "", tags: str = "", user_difficulty: str = "",
+                       match: str = "", genre: str = "", after: str = "", group: int = 0,
+                       naming_mode: str = "legacy"):
+    """Paginated library search through the selected library provider.
+
+    `after` is an opaque keyset cursor (feedBack#636 item 3): pass back the
+    `next_cursor` from the previous response to fetch the next page with a
+    WHERE-seek instead of OFFSET. Providers that don't support it ignore it and
+    page by OFFSET, so the client can always fall back."""
+    size = min(size, 100)
+    library_provider = _get_library_provider(provider)
+    _require_library_provider_capability(library_provider, "library.read")
+    # Only the true local provider keysets: it's the one whose effective sort is
+    # exactly the request `sort`. A smart collection may pin its own sort and
+    # remote providers don't keyset — both must page by OFFSET, so never hand
+    # them a cursor (a mismatched one would mis-seek).
+    is_local = getattr(library_provider, "id", "") == "local"
+    songs, total = await _call_library_provider_async(
+        library_provider,
+        "query_page",
+        page=page,
+        size=size,
+        sort=sort,
+        direction=dir,
+        after=((after or None) if is_local else None),
+        group=bool(group),
+        naming_mode=naming_mode,
+        mastery=_split_csv(mastery),
+        tags_has=_split_csv(tags),
+        user_difficulty_in=_split_csv(user_difficulty),
+        match_states=_split_csv(match),
+        genre=_split_csv(genre),
+        **_library_filter_args(
+            q=q, favorites=favorites, format=format,
+            artist=artist, album=album,
+            arrangements_has=arrangements_has, arrangements_lacks=arrangements_lacks,
+            stems_has=stems_has, stems_lacks=stems_lacks,
+            has_lyrics=has_lyrics, tunings=tunings,
+        ),
+    )
+    # The cursor to resume after this page (effective sort folds in dir=desc).
+    next_cursor = (next_library_cursor(_effective_keyset_sort(sort, dir), songs[-1])
+                   if (is_local and songs) else None)
+    # Drop the private raw-title stash query_page attached for the cursor — it's
+    # an internal keyset detail, not part of the card payload.
+    for s in songs:
+        s.pop("_sort_title", None)
+    return {"songs": songs, "total": total, "page": page, "size": size,
+            "next_cursor": next_cursor}
+
+
+@router.get("/api/library/albums")
+async def list_library_albums(q: str = "", page: int = 0, size: int = 120,
+                              favorites: int = 0, format: str = "",
+                              artist: str = "", album: str = "",
+                              arrangements_has: str = "", arrangements_lacks: str = "",
+                              stems_has: str = "", stems_lacks: str = "",
+                              has_lyrics: str = "", tunings: str = "", mastery: str = "",
+                              match: str = "", genre: str = "",
+                              provider: str = "local"):
+    """Album-condensed browse: distinct (artist, album) groups with a track count
+    and a representative cover song. Paged by album. Same filters as /api/library."""
+    size = min(size, 500)
+    library_provider = _get_library_provider(provider)
+    _require_library_provider_capability(library_provider, "library.read")
+    albums, total = await _call_library_provider_async(
+        library_provider, "query_albums",
+        page=page, size=size, mastery=_split_csv(mastery),
+        match_states=_split_csv(match), genre=_split_csv(genre),
+        **_library_filter_args(
+            q=q, favorites=favorites, format=format, artist=artist, album=album,
+            arrangements_has=arrangements_has, arrangements_lacks=arrangements_lacks,
+            stems_has=stems_has, stems_lacks=stems_lacks,
+            has_lyrics=has_lyrics, tunings=tunings,
+        ),
+    )
+    return {"albums": albums, "total": total, "page": page, "size": size}
+
+
+@router.get("/api/library/artists")
+async def list_artists(letter: str = "", q: str = "", favorites: int = 0, page: int = 0,
+                       size: int = 50, format: str = "",
+                       artist: str = "", album: str = "",
+                       arrangements_has: str = "", arrangements_lacks: str = "",
+                       stems_has: str = "", stems_lacks: str = "",
+                       has_lyrics: str = "", tunings: str = "", provider: str = "local",
+                       naming_mode: str = "legacy"):
+    """Get artists grouped by letter with albums and songs (for tree view)."""
+    size = min(size, 100)
+    library_provider = _get_library_provider(provider)
+    _require_library_provider_capability(library_provider, "library.read")
+    artists, total = await _call_library_provider_async(
+        library_provider,
+        "query_artists",
+        letter=letter,
+        page=page,
+        size=size,
+        naming_mode=naming_mode,
+        **_library_filter_args(
+            q=q, favorites=favorites, format=format,
+            artist=artist, album=album,
+            arrangements_has=arrangements_has, arrangements_lacks=arrangements_lacks,
+            stems_has=stems_has, stems_lacks=stems_lacks,
+            has_lyrics=has_lyrics, tunings=tunings,
+        ),
+    )
+    return {"artists": artists, "total_artists": total, "page": page, "size": size}
+
+
+@router.get("/api/library/stats")
+async def library_stats(favorites: int = 0, q: str = "", format: str = "",
+                        artist: str = "", album: str = "",
+                        arrangements_has: str = "", arrangements_lacks: str = "",
+                        stems_has: str = "", stems_lacks: str = "",
+                        has_lyrics: str = "", tunings: str = "", provider: str = "local",
+                        match: str = "",
+                        sort: str = "artist", sort_letters: int = 0,
+                        group: int = 0, naming_mode: str = "legacy"):
+    """Aggregate stats for the UI. Accepts the same filter params as
+    /api/library so the letter bar mirrors the active grid filter set.
+    `sort` selects the column the jump rail's `sort_letters` keys on;
+    `sort_letters=1` opts into that breakdown (the rail), so non-rail
+    callers skip the extra per-letter aggregate. `group=1` counts works not
+    charts (mirrors the grouped grid)."""
+    library_provider = _get_library_provider(provider)
+    _require_library_provider_capability(library_provider, "library.read")
+    return await _call_library_provider_async(
+        library_provider,
+        "query_stats",
+        naming_mode=naming_mode,
+        sort=sort,
+        want_sort_letters=bool(sort_letters),
+        group=bool(group),
+        # The match facet rides the stats call too — the A–Z rail's letter
+        # counts must agree with the grid under the facet or its cumulative
+        # seek + sizer geometry break.
+        match_states=_split_csv(match),
+        **_library_filter_args(
+            q=q, favorites=favorites, format=format,
+            artist=artist, album=album,
+            arrangements_has=arrangements_has, arrangements_lacks=arrangements_lacks,
+            stems_has=stems_has, stems_lacks=stems_lacks,
+            has_lyrics=has_lyrics, tunings=tunings,
+        ),
+    )
+
+
+@router.get("/api/library/genres")
+def library_genres(provider: str = "local"):
+    """Distinct non-empty genres for the filter facet.
+
+    Genres are a local-library facet: they're populated from the feedpak
+    `genres` field at scan time and live in the local meta DB. Local-backed
+    providers (the local library and its smart collections, kind="local")
+    share that DB, so they surface the same set. Remote providers don't
+    expose genres here, so return an empty facet for them — the client then
+    hides the filter rather than offering local genres that don't apply to
+    the remote grid. Mirrors the local/remote gating used elsewhere for
+    provider calls (see `_call_library_provider`)."""
+    library_provider = _get_library_provider(provider)
+    kind = str(appstate.library_providers.provider_field(library_provider, "kind", "") or "")
+    is_remote = kind not in ("", "local") if kind else provider != "local"
+    if is_remote:
+        return {"genres": []}
+    with appstate.meta_db._lock:
+        g = appstate.meta_db._effective_genre_expr()
+        rows = appstate.meta_db.conn.execute(
+            f"SELECT g FROM (SELECT DISTINCT ({g}) AS g FROM songs) "
+            "WHERE g IS NOT NULL AND g != '' ORDER BY g COLLATE NOCASE"
+        ).fetchall()
+    return {"genres": [r[0] for r in rows]}
+
+
+@router.get("/api/library/tuning-names")
+async def list_tuning_names(provider: str = "local"):
+    """Distinct tuning names present in the library, with per-tuning
+    counts. Powers the tuning multi-select. Sorted by `tuning_sort_key`
+    so names appear in the same musical order the sort uses
+    (feedBack#22) — E Standard first, then nearest neighbors."""
+    library_provider = _get_library_provider(provider)
+    _require_library_provider_capability(library_provider, "library.read")
+    return await _call_library_provider_async(library_provider, "tuning_names")
+
+
+@router.get("/api/library/practice-suggestions")
+def api_practice_suggestions(limit: int = 8):
+    """Growth-edge 'practice next' shelf (P3): attempted-but-not-mastered songs
+    ranked by difficulty-appropriateness × mastery-proximity, joined to song
+    metadata. Replaces the recency-only 'Keep practicing' shelf ordering. Local
+    library only — reads local practice stats."""
+    from urllib.parse import quote
+    out = []
+    for r in appstate.meta_db.growth_edge_suggestions(limit):
+        meta = appstate.meta_db.conn.execute(
+            "SELECT title, artist, tuning_name FROM songs WHERE filename = ?",
+            (r["filename"],),
+        ).fetchone()
+        title, artist, tuning_name = meta if meta else (None, None, None)
+        out.append({
+            **r,
+            "title": title or r["filename"],
+            "artist": artist or "",
+            "tuning_name": tuning_name or "",
+            "art_url": f"/api/song/{quote(r['filename'])}/art",
+        })
+    return out
+
+
+@router.get("/api/collections")
+def api_list_collections():
+    """Smart/dynamic collections (saved live library filters)."""
+    return {"collections": appstate.meta_db.list_collections()}
+
+
+@router.post("/api/collections")
+def api_create_collection(data: dict):
+    """Create a collection from a name + a set of library filter rules. It
+    immediately appears as a source in the library provider picker."""
+    if not isinstance(data, dict):
+        return JSONResponse({"error": "body must be an object"}, status_code=400)
+    name = _clean_str(data.get("name"))
+    if not name:
+        return JSONResponse({"error": "name required"}, status_code=400)
+    col = appstate.meta_db.create_collection(name, _sanitize_collection_rules(data.get("rules")))
+    _sync_collection_provider(col)
+    return {"ok": True, "collection": col}
+
+
+@router.put("/api/collections/{pid}")
+def api_update_collection(pid: int, data: dict):
+    """Rename a collection and/or replace its rules."""
+    if not isinstance(data, dict):
+        return JSONResponse({"error": "body must be an object"}, status_code=400)
+    name = _clean_str(data.get("name")) or None
+    rules = _sanitize_collection_rules(data["rules"]) if "rules" in data else None
+    col = appstate.meta_db.update_collection(pid, name=name, rules=rules)
+    if col is None:
+        return JSONResponse({"error": "collection not found"}, status_code=404)
+    _sync_collection_provider(col)
+    return {"ok": True, "collection": col}
+
+
+@router.delete("/api/collections/{pid}")
+def api_delete_collection(pid: int):
+    """Delete a collection and unregister its provider."""
+    if not appstate.meta_db.is_collection(pid):
+        return JSONResponse({"error": "collection not found"}, status_code=404)
+    appstate.meta_db.delete_playlist(pid)
+    _unregister_collection_provider(pid)
+    return {"ok": True}

static/app.js

@@ -5375,15 +5375,94 @@ window.jucePlayer = jucePlayer;
         if (_elCtx) return;
         const el = document.getElementById('audio');
         if (!el) throw new Error('no core audio element');
-        _elCtx = new AudioContext();
-        _elSource = _elCtx.createMediaElementSource(el);
-        _elSource.connect(_elCtx.destination);
-        _elTap = _makeTap(_elCtx);
-        await _elTap.attach(_elSource);
+        // 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' | 'element' | 'stems'
+    // '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;
@@ -5408,7 +5487,9 @@ window.jucePlayer = jucePlayer;
         const prev = _mode;
         _mode = 'off';
         try { await api.setRendererBus(false, 0); } catch (_) { /* engine gone */ }
-        if (prev === 'element' && _elCtx) {
+        if (prev === 'loopback') {
+            await _teardownLoopback();
+        } else if (prev === 'element' && _elCtx) {
             _elTap.active = false;
             await _setSink(_elCtx, false).catch(() => {});
         } else if (prev === 'stems' && _stemsGraph) {
@@ -5467,9 +5548,20 @@ window.jucePlayer = jucePlayer;
 
             let want = 'off';
             if (running && exclusive) {
-                if (stems) want = 'stems';
+                // 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
@@ -5481,6 +5573,7 @@ window.jucePlayer = jucePlayer;
                     + ' stems=' + !!stems + ' songAudio=' + !!songAudio
                     + ' juceMode=' + !!window._juceMode
                     + ' elementSong=' + elementSong
+                    + ' loopbackUnavailable=' + _loopbackUnavailable
                     + ' want=' + want + ' mode=' + _mode;
                 if (d !== window._lastRendererBusDecision) {
                     window._lastRendererBusDecision = d;
@@ -5492,12 +5585,29 @@ window.jucePlayer = jucePlayer;
             const stemsGraphChanged = _mode === 'stems' && stems !== _stemsGraph;
             if (want !== _mode || stemsGraphChanged) {
                 await _disengage();
-                if (want === 'stems') await _engageStems(stems);
-                else if (want === 'element') await _engageElement();
+                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) {
             console.warn('[renderer-bus] reevaluate failed (will retry):', e);
             _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;
         }

tests/js/renderer_bus_feeder.test.js

@@ -50,17 +50,42 @@ function makeFakeContext(sampleRate = 48000) {
             this.mediaSourceEl = el;
             return { connect() {}, disconnect() {} };
         },
+        createMediaStreamSource(stream) {
+            this.mediaStreamSource = stream;
+            return { connect() {}, disconnect() {} };
+        },
+        close() { this.closed = true; return Promise.resolve(); },
     };
     return ctx;
 }
 
-function makeSandbox({ isAudioRunning = () => true, exclusive = () => true } = {}) {
-    const calls = { setRendererBus: [], pushRendererAudio: [] };
+// Fake getDisplayMedia stream for the loopback-capture path.
+function makeLoopbackStream({ suppressed = true } = {}) {
+    const stopped = [];
+    const audioTrack = {
+        kind: 'audio',
+        stop() { stopped.push('audio'); },
+        getSettings: () => (suppressed ? { suppressLocalAudioPlayback: true } : {}),
+    };
+    const videoTrack = { kind: 'video', stop() { stopped.push('video'); } };
+    return {
+        __stopped: stopped,
+        getAudioTracks: () => [audioTrack],
+        getVideoTracks: () => [videoTrack],
+        getTracks: () => [videoTrack, audioTrack],
+    };
+}
+
+// `displayMedia`: undefined → loopback capture unavailable (Docker sphere /
+// old desktop main); a function → used as navigator.mediaDevices.getDisplayMedia.
+function makeSandbox({ isAudioRunning = () => true, exclusive = () => true, displayMedia } = {}) {
+    const calls = { setRendererBus: [], pushRendererAudio: [], setPageMuted: [] };
 
     const api = {
         isAudioRunning: () => Promise.resolve(isAudioRunning()),
         setRendererBus: (en, g) => { calls.setRendererBus.push([en, g]); return Promise.resolve(); },
         pushRendererAudio: (buf, rate) => { calls.pushRendererAudio.push([buf.length, rate]); },
+        setPageMuted: (m) => { calls.setPageMuted.push(m); return Promise.resolve(m); },
     };
 
     class FakeWorkletNode {
@@ -85,6 +110,7 @@ function makeSandbox({ isAudioRunning = () => true, exclusive = () => true } = {
         __createdContexts: [],
         __audioEl: { id: 'audio' },
         __calls: calls,
+        navigator: { mediaDevices: displayMedia ? { getDisplayMedia: displayMedia } : {} },
         window: null,
     };
     sandbox.window = {
@@ -111,12 +137,21 @@ function makeStemsGraph() {
     };
 }
 
-test('stems graph + exclusive output → bus enabled, stems ctx null-sinked', async () => {
+// Surface-mode (stems/element) tests run WITHOUT getDisplayMedia: the first
+// tick probes loopback, fails, and latches _loopbackUnavailable; the second
+// tick exercises the fallback surface mode. This mirrors an old desktop main
+// without the display-media handler.
+async function reevaluateWithFallback(sb) {
+    await sb.window._reevaluateRendererBus();   // loopback probe → unavailable
+    await sb.window._reevaluateRendererBus();   // surface fallback
+}
+
+test('stems graph + exclusive output → bus enabled, stems ctx null-sinked (loopback unavailable)', async () => {
     const sb = makeSandbox({ exclusive: () => true });
     const graph = makeStemsGraph();
     sb.window.feedBack.stems.audioGraph = graph;
 
-    await sb.window._reevaluateRendererBus();
+    await reevaluateWithFallback(sb);
 
     assert.deepEqual(sb.__calls.setRendererBus.at(-1), [true, 1.0], 'bus enabled');
     assert.equal(graph.context.sinkIdCalls.at(-1)?.type, 'none', 'stems ctx re-pointed at null sink');
@@ -128,7 +163,7 @@ test('output returns to shared → bus disabled, sink restored', async () => {
     const graph = makeStemsGraph();
     sb.window.feedBack.stems.audioGraph = graph;
 
-    await sb.window._reevaluateRendererBus();
+    await reevaluateWithFallback(sb);
     excl = false;
     await sb.window._reevaluateRendererBus();
 
@@ -145,26 +180,27 @@ test('stems graph + shared output → feeder stays off (no double audio)', async
     assert.equal(sb.__calls.setRendererBus.length, 0, 'bus never touched in shared mode');
 });
 
-test('element song + exclusive → element captured into bus', async () => {
+test('element song + exclusive → element captured into bus (loopback unavailable)', async () => {
     const sb = makeSandbox({ exclusive: () => true });
     sb.window._currentSongAudio = { url: '/api/sloppak/x.sloppak/file/stems/full.ogg' };
     sb.window._juceMode = false;
 
-    await sb.window._reevaluateRendererBus();
+    await reevaluateWithFallback(sb);
 
     assert.equal(sb.__createdContexts.length, 1, 'capture context created');
     assert.equal(sb.__createdContexts[0].mediaSourceEl, sb.__audioEl, 'element source captured');
     assert.deepEqual(sb.__calls.setRendererBus.at(-1), [true, 1.0], 'bus enabled');
 });
 
-test('song riding the native transport (_juceMode) → feeder stays off', async () => {
+test('native-transport song, loopback unavailable → surface modes stay off', async () => {
     const sb = makeSandbox({ exclusive: () => true });
     sb.window._currentSongAudio = { url: '/audio/song.ogg' };
     sb.window._juceMode = true;
 
-    await sb.window._reevaluateRendererBus();
+    await reevaluateWithFallback(sb);
 
-    assert.equal(sb.__calls.setRendererBus.length, 0, 'native transport owns the song');
+    assert.ok(!sb.__calls.setRendererBus.some(([en]) => en === true),
+        'bus never ENABLED (failed-probe cleanup may disable it)');
     assert.equal(sb.__createdContexts.length, 0, 'no capture context created');
 });
 
@@ -172,7 +208,7 @@ test('stems graph replaced mid-engagement → re-engages on the new graph', asyn
     const sb = makeSandbox({ exclusive: () => true });
     const g1 = makeStemsGraph();
     sb.window.feedBack.stems.audioGraph = g1;
-    await sb.window._reevaluateRendererBus();
+    await reevaluateWithFallback(sb);
 
     const g2 = makeStemsGraph();
     sb.window.feedBack.stems.audioGraph = g2;
@@ -182,6 +218,105 @@ test('stems graph replaced mid-engagement → re-engages on the new graph', asyn
     assert.deepEqual(sb.__calls.setRendererBus.at(-1), [true, 1.0], 're-enabled for new graph');
 });
 
+// ── Loopback mode (whole-app capture) ────────────────────────────────────────
+
+test('exclusive output + loopback available → engages without any song loaded', async () => {
+    const stream = makeLoopbackStream();
+    const sb = makeSandbox({ exclusive: () => true, displayMedia: () => Promise.resolve(stream) });
+
+    await sb.window._reevaluateRendererBus();
+
+    assert.deepEqual(sb.__calls.setRendererBus.at(-1), [true, 1.0], 'bus enabled for whole session');
+    assert.ok(stream.__stopped.includes('video'), 'unused video track stopped');
+    assert.equal(sb.__createdContexts.at(-1)?.mediaStreamSource, stream, 'loopback stream captured');
+    assert.equal(sb.__calls.setPageMuted.length, 0, 'suppress constraint honoured — no page mute');
+});
+
+test('loopback context is closed on disengage (no orphaned tap worklet)', async () => {
+    let excl = true;
+    const stream = makeLoopbackStream();
+    const sb = makeSandbox({ exclusive: () => excl, displayMedia: () => Promise.resolve(stream) });
+
+    await sb.window._reevaluateRendererBus();          // engage loopback
+    const lbCtx = sb.__createdContexts.at(-1);
+    assert.equal(lbCtx?.mediaStreamSource, stream, 'loopback engaged');
+    assert.notEqual(lbCtx.closed, true, 'context live while engaged');
+
+    excl = false;
+    await sb.window._reevaluateRendererBus();          // disengage
+    assert.equal(lbCtx.closed, true, 'loopback context closed on disengage');
+    assert.ok(stream.__stopped.includes('audio'), 'capture stream stopped');
+});
+
+test('loopback preferred over stems when both available', async () => {
+    const stream = makeLoopbackStream();
+    const sb = makeSandbox({ exclusive: () => true, displayMedia: () => Promise.resolve(stream) });
+    const graph = makeStemsGraph();
+    sb.window.feedBack.stems.audioGraph = graph;
+
+    await sb.window._reevaluateRendererBus();
+
+    assert.equal(graph.context.sinkIdCalls.length, 0, 'stems ctx untouched — loopback owns capture');
+    assert.equal(sb.__createdContexts.at(-1)?.mediaStreamSource, stream, 'loopback engaged');
+});
+
+test('suppressLocalAudioPlayback unsupported → page-mute fallback, unmuted on disengage', async () => {
+    let excl = true;
+    const stream = makeLoopbackStream({ suppressed: false });
+    const sb = makeSandbox({ exclusive: () => excl, displayMedia: () => Promise.resolve(stream) });
+
+    await sb.window._reevaluateRendererBus();
+    assert.deepEqual(sb.__calls.setPageMuted, [true], 'page muted as fallback');
+
+    excl = false;
+    await sb.window._reevaluateRendererBus();
+    assert.deepEqual(sb.__calls.setPageMuted, [true, false], 'page unmuted on disengage');
+    assert.deepEqual(sb.__calls.setRendererBus.at(-1), [false, 0], 'bus disabled');
+});
+
+test('getDisplayMedia rejected → sticky fallback to surface modes', async () => {
+    const sb = makeSandbox({
+        exclusive: () => true,
+        displayMedia: () => Promise.reject(new DOMException('denied', 'NotAllowedError')),
+    });
+    const graph = makeStemsGraph();
+    sb.window.feedBack.stems.audioGraph = graph;
+
+    await sb.window._reevaluateRendererBus();   // probe fails, latches unavailable
+    await sb.window._reevaluateRendererBus();   // falls back to stems
+
+    assert.equal(graph.context.sinkIdCalls.at(-1)?.type, 'none', 'stems fallback engaged');
+    assert.deepEqual(sb.__calls.setRendererBus.at(-1), [true, 1.0], 'bus enabled via fallback');
+});
+
+test('element capture collision (createMediaElementSource throws) → no poisoned state, clean retry', async () => {
+    const sb = makeSandbox({ exclusive: () => true });   // loopback unavailable
+    sb.window._currentSongAudio = { url: '/api/sloppak/x.sloppak/file/stems/full.ogg' };
+    // First capture attempt collides (highway analyser owns the element).
+    let collide = true;
+    const origFactory = sb.AudioContext;
+    sb.__createdContexts.length = 0;
+    // Patch contexts so createMediaElementSource throws while colliding.
+    sb.AudioContext = function () {
+        const c = origFactory();
+        const orig = c.createMediaElementSource.bind(c);
+        c.createMediaElementSource = (el) => {
+            if (collide) throw new DOMException('already connected', 'InvalidStateError');
+            return orig(el);
+        };
+        c.close = () => Promise.resolve();
+        return c;
+    };
+
+    await reevaluateWithFallback(sb);            // element engage fails (collision)
+    assert.ok(!sb.__calls.setRendererBus.some(([en]) => en === true), 'bus never left enabled');
+
+    collide = false;
+    await sb.window._reevaluateRendererBus();    // retry succeeds — no TypeError, fresh ctx
+
+    assert.deepEqual(sb.__calls.setRendererBus.at(-1), [true, 1.0], 'element engaged after collision cleared');
+});
+
 test('engine stops → bus disabled', async () => {
     let running = true;
     const sb = makeSandbox({ isAudioRunning: () => running, exclusive: () => true });

tests/test_collections_api.py

@@ -7,6 +7,7 @@ result, not stored songs.
 """
 
 import importlib
+import library_registry
 import sys
 
 import pytest
@@ -133,7 +134,7 @@ def test_collection_tolerates_corrupt_persisted_rules(client, server_mod):
         ('{"artist": [], "sort": [], "tunings": ["Drop D"]}', cid),
     )
     server_mod.meta_db.conn.commit()
-    server_mod._sync_collection_provider(server_mod.meta_db.get_collection(cid))
+    library_registry._sync_collection_provider(server_mod.meta_db.get_collection(cid))
 
     r = client.get("/api/library", params={"provider": f"collection:{cid}"})
     assert r.status_code == 200                          # no 500/503 from bad rules