refactor(app): carve the JUCE/desktop audio shims out of app.js (R3a)

static/js/juce-audio.js (994) — bodies VERBATIM. app.js 7,603 → 6,643.
THE LARGEST SINGLE SLICE of the whole carve phase: 960 lines, ~13% of what was left.

Three self-installing IIFEs:
  _installJuceEngineRoutingWatcher (444)  routes a song to the JUCE engine or HTML5 as
                                          the desktop output enters/leaves exclusive/ASIO
  _installRendererBusFeeder        (337)  feeds the highway renderer bus from whichever
                                          transport is actually running
  _installJuceAudioElementShim     (156)  patches audio.play/pause so the rest of the app
                                          keeps talking to the <audio> element while JUCE
                                          owns the transport

They EXPORT NOTHING — all three publish through `window.*` (_juceMode,
_reevaluateJuceRouting, _reevaluateRendererBus, …). So app.js needs only a
side-effect import plus the one binding it actually uses
(_resetJuceAudioShimChain, which the shim IIFE assigns).

THE ORDERING QUESTION, CHECKED RATHER THAN ASSUMED. Importing this module runs the
IIFEs EARLIER than before: imports evaluate ahead of app.js's body, and therefore
ahead of configureHost(). A hook read at IIFE-execution time would THROW. So I walked
the AST at IIFE-body depth to see what they actually touch when they run: nothing but
listener registration, and `audio.play`/`audio.pause` patching — and `audio` is itself
an imported module now. Verified in the browser: both are patched on the carved build
exactly as on main, which proves the shim installs correctly at its new, earlier point.
(Had I got this wrong, host.js throws loudly rather than silently misbehaving — which
is the whole reason it has no no-op defaults.)

VERIFIED. A/B against origin/main in two browsers: the entire window.* surface the
IIFEs publish (_juceMode, _juceOutputIsExclusive, _reevaluateJuceRouting,
_reevaluateRendererBus, _clearJuceRerouteMemo), audio.play/pause patched, a real song
loading and togglePlay driving the public mirror — IDENTICAL, zero page errors.

Harnesses: juce_engine_reroute (19 tests) + renderer_bus_feeder (13) slice the IIFEs by
signature — retargeted, and each sandbox gains a `host` object routed at its EXISTING
stubs so every assertion holds unchanged. test_plugin_runtime_idempotence is SPLIT: 3 of
its 4 source-asserts stayed in app.js, the sm.emit('song:resume') one moved.

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

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

tests/js/juce_engine_reroute.test.js

@@ -1,4 +1,4 @@
-// Behavioral tests for the JUCE engine-reroute watcher in static/app.js.
+// Behavioral tests for the JUCE engine-reroute watcher in static/js/juce-audio.js.
 //
 // The watcher (an IIFE, `_installJuceEngineRoutingWatcher`) migrates a loaded
 // song between the HTML5 <audio> element and the native JUCE backing transport
@@ -14,14 +14,15 @@ const fs = require('node:fs');
 const path = require('node:path');
 const vm = require('node:vm');
 
-const APP_JS = path.join(__dirname, '..', '..', 'static', 'app.js');
+// The JUCE audio shims were carved out of app.js into their own module (R3a).
+const APP_JS = path.join(__dirname, '..', '..', 'static', 'js', 'juce-audio.js');
 
 // Brace-balanced extraction of the watcher IIFE, starting at its `(function`
 // and ending after the matching `})();`.
 function extractWatcherIIFE(src) {
     const marker = '(function _installJuceEngineRoutingWatcher() {';
     const start = src.indexOf(marker);
-    assert.ok(start !== -1, 'watcher IIFE not found in app.js');
+    assert.ok(start !== -1, 'watcher IIFE not found in static/js/juce-audio.js');
     const openBrace = src.indexOf('{', start);
     let depth = 1;
     let i = openBrace + 1;
@@ -100,6 +101,17 @@ function makeSandbox({ isAudioRunning, loadBackingTrack, outputType = 'Windows A
 
     const src = fs.readFileSync(APP_JS, 'utf8');
     const iife = extractWatcherIIFE(src);
+    // The shims reach back into app.js through the host seam (static/js/host.js).
+    // Route it at the SAME stubs this sandbox already had — a fresh `() => {}` would
+    // swallow the calls and the assertions below would pass vacuously.
+    sandbox.host = {
+        jucePlayer: () => sandbox.jucePlayer,
+        playSong: (...a) => (sandbox.playSong ? sandbox.playSong(...a) : undefined),
+        _audioSeek: (...a) => (sandbox._audioSeek ? sandbox._audioSeek(...a) : Promise.resolve({ completed: true })),
+        setPlayButtonState: (...a) => (sandbox.setPlayButtonState ? sandbox.setPlayButtonState(...a) : undefined),
+        _songEventPayload: (...a) => (sandbox._songEventPayload ? sandbox._songEventPayload(...a) : ({})),
+        showScreen: (...a) => (sandbox.showScreen ? sandbox.showScreen(...a) : undefined),
+    };
     vm.createContext(sandbox);
     vm.runInContext(iife, sandbox);
     return sandbox;

tests/js/renderer_bus_feeder.test.js

@@ -1,4 +1,4 @@
-// Behavioral tests for the renderer-audio bus feeder in static/app.js.
+// Behavioral tests for the renderer-audio bus feeder in static/js/juce-audio.js.
 //
 // The feeder (an IIFE, `_installRendererBusFeeder`) captures renderer-side
 // song audio (stems-plugin WebAudio master, or the core <audio> element) and
@@ -16,12 +16,13 @@ const fs = require('node:fs');
 const path = require('node:path');
 const vm = require('node:vm');
 
-const APP_JS = path.join(__dirname, '..', '..', 'static', 'app.js');
+// The JUCE audio shims were carved out of app.js into their own module (R3a).
+const APP_JS = path.join(__dirname, '..', '..', 'static', 'js', 'juce-audio.js');
 
 function extractFeederIIFE(src) {
     const marker = '(function _installRendererBusFeeder() {';
     const start = src.indexOf(marker);
-    assert.ok(start !== -1, 'feeder IIFE not found in app.js');
+    assert.ok(start !== -1, 'feeder IIFE not found in static/js/juce-audio.js');
     const openBrace = src.indexOf('{', start);
     let depth = 1;
     let i = openBrace + 1;
@@ -123,6 +124,17 @@ function makeSandbox({ isAudioRunning = () => true, exclusive = () => true, disp
     sandbox.globalThis = sandbox;
 
     const src = fs.readFileSync(APP_JS, 'utf8');
+    // The shims reach back into app.js through the host seam (static/js/host.js).
+    // Route it at the SAME stubs this sandbox already had — a fresh `() => {}` would
+    // swallow the calls and the assertions below would pass vacuously.
+    sandbox.host = {
+        jucePlayer: () => sandbox.jucePlayer,
+        playSong: (...a) => (sandbox.playSong ? sandbox.playSong(...a) : undefined),
+        _audioSeek: (...a) => (sandbox._audioSeek ? sandbox._audioSeek(...a) : Promise.resolve({ completed: true })),
+        setPlayButtonState: (...a) => (sandbox.setPlayButtonState ? sandbox.setPlayButtonState(...a) : undefined),
+        _songEventPayload: (...a) => (sandbox._songEventPayload ? sandbox._songEventPayload(...a) : ({})),
+        showScreen: (...a) => (sandbox.showScreen ? sandbox.showScreen(...a) : undefined),
+    };
     vm.createContext(sandbox);
     vm.runInContext(extractFeederIIFE(src), sandbox);
     assert.equal(typeof sandbox.window._reevaluateRendererBus, 'function',