refactor(app): lift the shared player state onto a container (R3a) (#889)

static/js/player-state.js — one exported object, two fields. app.js's 70 reference
sites rewritten. Provably a no-op; nothing shrinks.

WHY NOW. Every slice carved out of app.js so far only ever READ the state it shared
(loopA/loopB, _audioSeekGen, currentFilename), so a read-only getter hook was enough
and no container was needed — twice I checked and twice I got away with it. That
runs out at count-in: it genuinely WRITES `isPlaying` (it starts and stops playback,
4 sites) and `lastAudioTime` (2). `import { isPlaying }` then `isPlaying = true`
THROWS — an imported binding cannot be assigned to. So the state has to live on an
object: `S.isPlaying = true` is a property write, and works from any module holding
the same S. Same shape stems, studio, and editor all converged on.

DELIBERATELY SMALL. app.js has ~104 top-level `let` scalars; lifting all of them is
a ~977-site rewrite for no benefit, because most are private to one cluster and
travel with it. Only what a carved module must WRITE goes here. Add on demand.

THE REWRITE IS AST-DRIVEN, NOT TEXTUAL — and that is not fussiness. Of 100 textual
occurrences of these two names, only 70 resolve to the module binding:
  * 22 are member accesses (`someObj.isPlaying`, `window.feedBack.isPlaying`)
  * 4 are the LOCAL PARAMETER of `function setPlayButtonState(isPlaying)` — a blind
    replace yields `function setPlayButtonState(S.isPlaying)`
  * 1 is an object key
  * 2 are shorthand properties `{ isPlaying }`, which must become
    `{ isPlaying: S.isPlaying }` — and acorn gives a shorthand's key and value the
    SAME range, so rewriting both produced `isPlaying: S.isPlaying: S.isPlaying`
    until I deduped by range
A find-and-replace corrupts all 29. The rewrite walks the AST, skips shadows, member
properties and keys, and replaces identifier RANGES.

`window.feedBack.isPlaying` — the PUBLIC mirror — is a different thing and is
untouched. Two test sandboxes stub it; those were left alone deliberately.

VERIFIED WITH REAL PLAYBACK. A/B against origin/main in two browsers, real song:
togglePlay -> the public mirror goes true -> false -> true across two toggles, the
audio element's paused state follows, seekBy works — IDENTICAL, zero page errors.

Harnesses: 8 vm-sandbox suites slice playback code out of app.js and now see
S.isPlaying — juce_engine_reroute, loop_restart, play_button_reroute_guard,
playback_app_adapter, song_restart, song_seek, speed_reset, and the python
idempotence source-assert. Each gets the same container in its sandbox; every
assertion is unchanged.

pytest 2396, node 1040/1040, ESLint 0, tailwind clean, Codex 0.

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

tests/js/play_button_reroute_guard.test.js

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