refactor(app): carve the playback transport out of app.js — and RETIRE 8 host hooks (R3a) (#894)

static/js/transport.js (377) — bodies VERBATIM. app.js 6,643 → 6,316.

THIS IS THE FIRST CARVE THAT SUBTRACTS HOOKS INSTEAD OF ADDING THEM.

Every carve before this one added host hooks: a module pulled out of app.js still had
to call back into it. But four modules 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
consumers import them directly:

    count-in.js           5 hooks -> 0     (host import deleted)
    juce-audio.js         4 hooks -> 0     (host import deleted)
    loops.js              6 hooks -> 4
    section-practice.js  10 hooks -> 7
    ----------------------------------------------------------
    configureHost()      20 hooks -> 12

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.

_audioSeekGen now stays PRIVATE. It has exactly one writer — _resetAudioSeekState(),
which moved with it — so readers get audioSeekGen() and nobody outside can desync it.
Strictly better than the hook it replaces, which handed out a getter and left the writer
behind in app.js.

THE SCAN HAD A HOLE, AND IT BIT. Picking the carve by dependency closure over app.js's
own top-level decls said this cluster was downward-closed. It wasn't:
_currentPlaybackSnapshot reads loopA/loopB — which live in ./js/loops.js, and loops.js
imports transport. The scan saw nothing, because loopA STOPPED BEING an app.js decl the
moment loops.js was carved out. Any dependency scan of a partly-carved monolith has to
resolve the imports too, or it will confidently hand you a cycle. Added that pass; it
found exactly one back-edge, and _currentPlaybackSnapshot stays in app.js (as does
restartCurrentSong, which calls _cancelCountIn). app.js is the root — it imports both
sides for free.

TESTS. Four harnesses retargeted (play_button_reroute_guard, song_event_payload,
song_seek -> transport.js; playback_app_adapter SPLIT, since
_installPlaybackTransportAdapter stayed behind).

The two CENSUS tests — "≥8 song:* emit sites", "every seek callsite passes a reason" —
now scan app.js AND every static/js/*.js, not one file. Pointed at a single file, their
count silently shrinks as code leaves, which reads as "someone deleted an emit" or, worse,
passes while genuinely missing sites. Both bite-tested: stripping a _songEventPayload()
from an emit and adding a reason-less _audioSeek() each fail the suite.

VERIFIED. A/B against origin/main, real song, real playback: song:play payload is exactly
{audioT, chartT, perfNow, time}; song:seek carries reason "seek-by" with finite from/to;
all five song:* events fire; seekBy advances the clock; restartCurrentSong returns to zero;
the play button's aria-pressed tracks state. IDENTICAL on all 21 probes, zero page errors.

pytest 2396, node 1040/1040, host contract 2/2, ESLint 0 (no-cycle clean), Codex 0.

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

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,