refactor(app): carve the viz layer out of app.js — and delete the loader seam (R3a) (#880)

static/js/viz.js (770 lines) — the viz picker, renderer selection, Auto-match,
the WebGL2 probe, the 3D-promotion nag, the notation hints. Bodies VERBATIM.
app.js 11,603 → 10,857.

THE SEAM IS GONE. #878's plugin-loader needed configurePluginLoader({
populateVizPicker }) purely because _populateVizPicker lived in app.js and
importing app.js would have closed a cycle. viz.js is a LEAF — it imports NOTHING
— so plugin-loader now imports _populateVizPicker straight from it. The _host
object, the configure function, its loud-default guard, and the wiring line in
app.js are all deleted. The second carve simplifies the first.

  app.js -> { plugin-loader, viz }
  plugin-loader -> viz
  viz -> (nothing)

NOT A PURE MOVE — one listener block had to be SPLIT. app.js had a single
top-level `if (window.feedBack) { … }` registering four handlers, and only two
were viz. song:loaded / arrangement:changed / song:ready (the mastery slider)
stay in app.js and now call the imported _autoMatchViz / _maybeShowNotationViewHint.
The viz:reverted handler MOVES, because it REASSIGNS _cancelPendingAutoLabel and
an imported binding is read-only — `_cancelPendingAutoLabel = null` would throw if
the listener stayed behind while the state moved.

ORDER CHECKED, NOT ASSUMED: viz.js's song:ready listener now registers BEFORE
app.js's own (imports evaluate first). Safe — _pendingPromotionNag is only ever
set inside _populateVizPicker, which runs at boot/plugin-refresh, never from
inside the other song:ready handler, so the two are independent.

VERIFIED — the listeners are the risk here, so they were DRIVEN, not just booted.
A/B against origin/main in two browsers:
  * viz picker: 6 options (auto|default|venue|drum_highway_3d|keys_highway_3d|
    highway_3d), selected highway_3d, Auto label — IDENTICAL. This alone proves
    plugin-loader's direct import of viz.js works.
  * emit('viz:reverted') -> picker resets to default, localStorage resets to
    default, the warning logs — IDENTICAL. The MOVED listener fires.
  * emit('song:ready') -> mastery slider enables, no throw — IDENTICAL. The SPLIT
    listener still does both halves.
  * plugin screens, module injections, 37 capability participants — IDENTICAL.
  * zero console/page errors on both.

pytest 2396, node 1038/1038, ESLint 0, tailwind-fresh clean. no-cycle re-bitten on
the 3-module graph (viz -> plugin-loader fails).

Codex preflight raised a [P2] claiming viz.js's top-level bus guards would be
false because "app.js only creates the event bus later" — FALSE POSITIVE. app.js
does not create the bus; capabilities.js does, from its own <script type="module">
at index.html:122, and module scripts execute in document order, so the bus exists
long before app.js's import graph evaluates. Instrumented the setter: by viz.js's
turn `window.feedBack.on` is already a function, and the viz:reverted listener is
provably attached (firing it resets the picker). The ordering is also enforced by
test_app_shell_loads_capability_registry_before_app_runtime.

Harnesses: 5 tests retargeted to viz.js across legacy_shim_hits, venue_scene_3d,
venue_viz (each SPLIT — their non-viz tests still read app.js).

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

tests/js/venue_scene_3d.test.js

@@ -9,6 +9,9 @@ const venueScene = require('../../static/v3/venue-scene-3d.js');
 const venueViz = require('../../static/v3/venue-viz.js');
 const pov = require('../../static/v3/venue-instrument-pov.js');
 const APP_JS = path.join(__dirname, '..', '..', 'static', 'app.js');
+// The viz layer (setViz / the venue option / the picker) was carved out of
+// app.js into its own module (R3a).
+const VIZ_JS = path.join(__dirname, '..', '..', 'static', 'js', 'viz.js');
 const H3D_JS = path.join(__dirname, '..', '..', 'plugins', 'highway_3d', 'screen.js');
 const INDEX_HTML = path.join(__dirname, '..', '..', 'static', 'v3', 'index.html');
 const ASSET_DIR = path.join(__dirname, '..', '..', 'static', 'assets', 'venue', 'themes', 'small-club');
@@ -185,8 +188,8 @@ test('venue-scene-3d exports bg plate asset ids', () => {
     assert.equal(venueScene.ASSET_BASE, '/static/assets/venue/themes/small-club/');
 });
 
-test('app.js syncs venue 3D scene on viz changes', () => {
-    const src = fs.readFileSync(APP_JS, 'utf8');
+test('viz.js syncs venue 3D scene on viz changes', () => {
+    const src = fs.readFileSync(VIZ_JS, 'utf8');
     assert.match(src, /v3VenueScene3d\.syncViz\('venue'\)/);
     assert.match(src, /v3VenueScene3d\.syncViz\(id\)/);
 });