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/play_button_reroute_guard.test.js

@@ -18,7 +18,7 @@ const vm = require('node:vm');
 
 const { extractFunction } = require('./test_utils');
 
-const APP_JS = path.join(__dirname, '..', '..', 'static', 'app.js');
+const APP_JS = path.join(__dirname, '..', '..', 'static', 'js', 'transport.js');
 const SRC = fs.readFileSync(APP_JS, 'utf8');
 const TOGGLE_PLAY_SRC = extractFunction(SRC, 'async function togglePlay(');