refactor(app): carve the song session out of app.js — playSong, showScreen, closeCurrentSong (R3d) (#921)

36 declarations + the 4 autoplay/auto-exit gate statements. 359 lines.
app.js 3,772 -> 3,242. Bodies VERBATIM.

━━━ THIS WAS "THE UNCUTTABLE HEART", AND IT IS 359 LINES ━━━

At the start of this epic, seeding a dependency closure from count-in, from loops, from
section-practice, or from the JUCE seek shim all returned the SAME 178-function, 3,360-line set.
playSong and showScreen called each other; everything called them; nothing could be cut anywhere.
The conclusion — correct at the time — was that NO closure-based carve could touch it at any
seed, and the answer was a host seam.

That was true THEN. Every slice taken out since (transport, loops, count-in, section-practice,
the library, the edit modal, settings) removed edges, and the strongly-connected component
DISSOLVED. This closure is 36 declarations with an interface width of FOUR.

The lesson is not that the seam was wrong — the seam is what MADE this possible, by letting the
carves proceed against a cyclic core instead of stalling on it. The lesson is to RE-MEASURE. An
SCC is a fact about a graph at a moment, not a property of the code.

━━━ THE BUG NO SCAN COULD SEE, AND THE A/B DID ━━━

First cut passed every gate — no-undef clean, no-cycle clean, 1045/1045, pytest green — and
THREW IN THE BROWSER: "Assignment to constant variable."

window.feedBack.holdAutoplay / holdAutoExit and their two event handlers are TOP-LEVEL
STATEMENTS, not declarations. They WRITE this cluster's state (_autoplayHeld, _autoExitTimer, …),
and an imported binding is READ-ONLY — so left behind in app.js, every one threw the instant the
module existed.

A dependency scan that walks DECLARATIONS cannot see them. Mine didn't. This is the same blind
spot that nearly shipped a dead library A-Z rail (#896): app.js keeps its public API in top-level
statements, and a call-graph is blind to every one of them.

The extractor now finds them by construction — any top-level statement that WRITES a moved
binding comes with the carve — and the gate statements live beside the machinery they drive,
which is where they belonged anyway.

━━━ ZERO OUTSIDE WRITES, BY MOVING THE BOUNDARY RATHER THAN BUILDING MACHINERY ━━━

The autoplay scalars and the wake-lock state were written from outside the cluster, which would
have forced a setter or a state container. But the writers — _releaseAutoplay, _acquireWakeLock —
plainly belong here. Pulling them in left ZERO outside writes, so every export is a plain import.
Same move as settings (#920): measure the writers before you reach for a container.

VERIFIED. A/B against origin/main in two browsers, IDENTICAL, zero page errors — including the
autoplay gate driven end to end: a plugin HOLDS autoplay, the song loads but does not start, the
RELEASE fires it, and a stale release is a no-op. That is the exact machinery that was throwing.

node 1045, pytest 2425, ESLint 0 (no-cycle clean), host contract 2/2, Codex 0.

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

tests/js/tuner_auto_open.test.js

@@ -488,8 +488,14 @@ test('gate: does not claim a hold when the feature is off', async () => {
     assert.equal(holds, 0);
 });
 
-test('the autoplay gate is a generic core hook with a fail-open backstop (app.js)', () => {
-    const appSrc = fs.readFileSync(APP_JS, 'utf8');
+test('the autoplay gate is a generic core hook with a fail-open backstop', () => {
+    // R3d: the gate SPANS two files now. window.feedBack.holdAutoplay is the public hook and
+    // stays on app.js's window contract; the machinery it drives (_autoplayHeld,
+    // _clearAutoplayHold, the backstop) moved to static/js/session.js with playSong. Read both —
+    // re-pinning at one would silently stop checking half the gate.
+    const SESSION_JS = path.join(__dirname, '..', '..', 'static', 'js', 'session.js');
+    const appSrc = fs.readFileSync(APP_JS, 'utf8')
+        + '\n' + fs.readFileSync(SESSION_JS, 'utf8').replace(/^export /gm, '');
     assert.match(appSrc, /window\.feedBack\.holdAutoplay = function/);
     assert.match(appSrc, /AUTOPLAY_HOLD_BACKSTOP_MS/);                       // fail-open: never strand the song
     assert.match(appSrc, /if \(_autoplayHeld\) \{ _autoplayStart = start;/); // a gated start is stashed