feat(drum_highway_3d): audio-reactive ambience + score FX overlay (D4) (#708)

- BG_STYLES port (off/particles/lights/geometric) into a renderOrder -1
  group; _bgGetAnalyser/_bgReadBands (stems-first, one-shot #audio
  fallback, permanent-failure latch, 5ms bands cache); Ambience
  intensity slider + Audio-reactive toggle; remounts on style/intensity/
  palette change and across kit-change scene rebuilds
- Score-FX overlay canvas (guitar drawScoreFx adapted to internal
  scoring): +1 pops at the struck lane, ring pulse every 10-combo,
  milestone bursts at 25/50/100, red wash on 3+ streak break; pooled,
  cleared when idle, removed in teardown
- butterchurn/image/video deliberately out of scope
- Tests: style id validation + FX defaults (15 total)

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

plugins/drum_highway_3d/README.md

@@ -82,6 +82,13 @@ tunes one of these, mirror the change here (and in `keys_highway_3d`):
   as the guitar's, except `default` which is this plugin's original
   palette); one pick drives both of the guitar's background/highway axes
 - `_applyCinematic()` — ambient/key rebalance (values tuned per plugin)
+- `BG_STYLES` (off/particles/lights/geometric) + `_bgGetAnalyser()` /
+  `_bgReadBands()` — background ambience + the stems-first audio-analyser
+  bridge (guitar's diagnostics plumbing dropped; butterchurn/image/video
+  out of scope)
+- `_drawScoreFx()` — the guitar's drawScoreFx overlay adapted to this
+  plugin's internal scoring (pops / tier rings / milestone bursts /
+  streak-break wash)
 
 ## Why a separate plugin (vs. drum mode inside highway_3d)?