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>

static/js/loops.js

@@ -19,6 +19,7 @@
 // See ./host.js: reading an unwired hook THROWS, and tests/js/host_contract.test.js
 // fails CI if the hooks used here and the hooks app.js wires ever drift apart.
 import { esc, uiPrompt } from './dom.js';
+import { _audioSeek, _audioTime } from './transport.js';
 import { host } from './host.js';
 import {
     _setSectionPracticeMode,
@@ -39,14 +40,14 @@ export let loopB = null;
 export let _loopMutationGen = 0;
 
 export function setLoopStart() {
-    loopA = host._audioTime();
+    loopA = _audioTime();
     document.getElementById('btn-loop-a').className = 'px-3 py-1.5 bg-green-900/50 rounded-lg text-xs text-green-300 transition';
     updateLoopUI();
 }
 
 export function setLoopEnd() {
     if (loopA === null) return;
-    loopB = host._audioTime();
+    loopB = _audioTime();
     if (loopB <= loopA) { loopB = null; return; }
     document.getElementById('btn-loop-b').className = 'px-3 py-1.5 bg-green-900/50 rounded-lg text-xs text-green-300 transition';
     updateLoopUI();
@@ -72,7 +73,7 @@ export function clearLoop(options) {
     document.getElementById('loop-label').textContent = '';
     document.getElementById('saved-loops').value = '';
     resetSelection();
-    _updateSectionPracticeHighlight(host._audioTime());
+    _updateSectionPracticeHighlight(_audioTime());
     if (hadLoop && emitTransportEvent && typeof window !== 'undefined') {
         window.feedBack?.playback?.transportEvent?.('loop-cleared', {
             requesterId: 'core.loop',
@@ -125,7 +126,7 @@ export async function setLoop(a, b, options) {
     // Don't arm loopA/loopB before the seek lands — the 60Hz tick's wrap
     // detector (`ct >= loopB`) would trigger startCountIn against
     // half-applied state.
-    const r = await host._audioSeek(aNum, 'loop-set');
+    const r = await _audioSeek(aNum, 'loop-set');
     if (!r.completed || Math.abs(r.to - aNum) > 0.05) return false;
     // Caller-owned staleness gate, re-checked after the awaited seek and before
     // we commit loopA/loopB. practiceSection() passes this so a superseded retry