feat(highway_3d): Butterchurn visualizer background style (#598)

* feat(highway_3d): add Butterchurn visualizer background style

Adds an opt-in "Butterchurn (visualizer)" option to the 3D Highway plugin's
Background-style dropdown. When selected, the highway renders over a WebGL
MilkDrop (Butterchurn) canvas that reacts to your playing (guitar input on
desktop, the song <audio> spectrum in the browser) and the chart (beat/note/
chord accents + instrument-color tint). The default stays 'particles', so
existing users see no change until they pick it.

Integrates the standalone "3D Highway + Butterchurn" mod into the bundled
renderer as the 'butterchurn' bg-style (not a fork):
- a self-contained _bc* controller that lazy-loads the vendored butterchurn
  libs only when the style is selected; mount/unmount is driven idempotently
  by the existing bg-style lifecycle (_bcSyncMode in _bgMountStyle) plus an
  explicit teardown in destroy()
- the renderer uses alpha:true with the transparent clear gated on the mode,
  so every other bg style stays byte-identical (opaque clear)
- the fog-scenery <audio> tap is disabled while active to avoid a double
  createMediaElementSource on #audio
- the mod's slopsmith* globals are adapted to the current feedBack* names and
  the vendored asset URLs repointed to /api/plugins/highway_3d/assets/

Vendors butterchurn.min.js + butterchurnPresets.min.js (MIT) + viz-worklet.js
under assets/vendor/; see plugins/highway_3d/NOTICE for attribution.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011Q9BpGYqUaga9ZJyS3dDPq

* fix(highway_3d): anchor Butterchurn panel to the highway, centered

Addresses three issues found testing the visualizer control panel:
- Attach the panel + preset pane to the 3D highway's `wrap` (position:absolute,
  pointer-events:auto) instead of position:fixed on document.body, so they sit
  on the highway's right edge and only exist while the highway is on-screen
  (no longer linger on the main menu / float at the app edge).
- Re-home the singleton panel to the active highway wrap on mount, so it follows
  whichever highway is showing (e.g. moves off Virtuoso's embedded highway onto a
  normal song's highway) instead of sticking to the first one created.
- Center it vertically (top:50% + translateY(-50%), folded into the slide
  transform) so a top overlay element no longer covers it.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011Q9BpGYqUaga9ZJyS3dDPq

* fix(highway_3d): harden Butterchurn audio + lifecycle (review #597)

Browser audio reactivity now REUSES the highway's existing shared analyser
(the fog scenery's #audio / stems side-chain tap) instead of opening a
second createMediaElementSource on #audio. The old _bcBrowserSource path:
  - threw InvalidStateError when the fog tap already owned #audio (default
    config), leaving the visualizer non-reactive in the browser, and could
    permanently disable fog reactivity if it tapped first (one-shot/element);
  - rerouted the song through a fresh, possibly-suspended AudioContext, which
    could MUTE playback when butterchurn was selected mid-song;
  - ignored the stems analyser, so it saw only silence on sloppak songs.
_bcCreateController now takes an audioProvider (wired to _bgGetAnalyser) and
connectAudio()s the shared AnalyserNode (a passthrough, so the fog's own
reads are undisturbed).

Also:
- destroy() now closes the AudioContext when we own it (desktop / browser
  fallback), fixing a per-mount leak that hit the browser ~6-context cap
  after a few style toggles. The shared (fog-owned) context is never closed.
- _bgApplyVenueSceneFog keeps the clear transparent while butterchurn is
  active, so the venue scene no longer occludes the visualizer.
- _bcLoadLib no longer caches a rejected promise, so a transient vendor-load
  failure can be retried instead of disabling the feature for the session.

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

* fix(highway_3d): Butterchurn lifecycle + audio re-bind (Codex preflight)

Local Codex preflight on the Butterchurn feature flagged four issues; all fixed:

- WebGL context leak on teardown: destroy() (and the async-init failure path)
  now call _bcReleaseCanvasGL() to force WEBGL_lose_context before dropping the
  canvas, so repeated mount/toggle cycles can't exhaust the browser's WebGL
  context cap.
- Stale shared analyser across songs: the browser path captured the analyser
  once at mount, so a sloppak stems swap (new analyser, often new context) left
  the visualizer reacting to a dead node. update() now compares the live
  _bgGetAnalyser() against what the controller actually bound (boundAnalyser(),
  guarded by ready()) and either reconnects (same context) or rebuilds the
  controller (context changed) via the proven destroy()+_bcSyncMode paths.
- Half-mounted controller on createVisualizer failure: the async .catch now
  cleans up (closes an owned AudioContext, removes layers, marks dead) and
  _bcSyncMode retries when bcCtrl.dead(), instead of leaking and never recovering.
- _bcFfIdx off-by-one dropped accents landing exactly on a seek/loop target
  time; it now uses strict < so the update walkers fire the boundary event.

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

---------

Co-authored-by: ChrisBeWithYou <christian.a.cowan@gmail.com>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

plugins/highway_3d/settings.html

@@ -124,6 +124,7 @@
             <option value="silhouettes">Silhouettes (parallax)</option>
             <option value="lights">Lights (stage glows)</option>
             <option value="geometric">Geometric (rotating shapes)</option>
+            <option value="butterchurn">Butterchurn (visualizer)</option>
             <option value="image" disabled>Custom image (none uploaded)</option>
             <option value="video" disabled>Custom video (none uploaded)</option>
         </select>
@@ -779,7 +780,7 @@
     <script>
         (function () {
             const DEFAULTS = { style: 'particles', intensity: 0.5, reactive: true, palette: 'default', bgTheme: 'default', hwTheme: 'default', showFretOnNote: true, fretNumberGhostScope: 'chords', cameraSmoothing: 0.5, zoomSmoothing: 0.5, tiltSmoothing: 0.5, cameraLockLow: false, cameraLockZoom: 0.5, cameraMode: 'lookahead', nutHeadstockVisible: true, tuningLabelsVisible: true, nutColor: '#f5f3f0', headstockColor: '#d4b48a', textSize: 0.5, vibrancy: 0.85, glow: 0.25, customImageDataUrl: '', customImageName: '', customVideoName: '', chordDiagramVisible: true, chordDiagramSize: 0.5, chordDiagramPosition: 'tl', fretColumnMarkerCadence: 1, projectionVisible: true, inlayLabelsVisible: false, sectionLabelsOnHighway: false, sectionHudVisible: false, sectionHudPosition: 'tr', sectionHudSize: 0.5, toneHudVisible: false, toneHudPosition: 'tl', toneHudSize: 0.5, fpsVisible: false, fretDividersVisible: true, slideArrowApproachVisible: true, slideArrowNeckVisible: true, slideArrowChainPreviewVisible: true };
-            const VALID_STYLES = new Set(['off', 'particles', 'silhouettes', 'lights', 'geometric', 'image', 'video']);
+            const VALID_STYLES = new Set(['off', 'particles', 'silhouettes', 'lights', 'geometric', 'butterchurn', 'image', 'video']);
             // Scene color themes — the single id+label source for BOTH the
             // Background and Highway dropdowns AND the validator below. The
             // <option>s are generated from this list (see hydration), so the two