feat(keys_highway_3d): audio-reactive ambience + score FX overlay (K4) (#709)

- BG_STYLES port (off/particles/lights/geometric; lights use the
  pitch-class palette) mounted behind the scene; _bgGetAnalyser/
  _bgReadBands (stems-first, one-shot #audio fallback, permanent-failure
  latch); Ambience intensity + Audio-reactive settings; remounts on
  style/intensity change
- Score-FX overlay canvas (drum_highway_3d pattern): +1 pops at the
  scored key, ring pulse every 10-combo, milestone bursts at 25/50/100,
  red wash on 3+ streak break (wrong notes AND swept misses); cleared
  when idle, removed in teardown
- Tests: style id validation + FX defaults (30 total)

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

plugins/keys_highway_3d/tests/fx_settings.test.js

@@ -133,3 +133,18 @@ test('FX defaults: theme-PR controls ship enabled at stock-neutral values', () =
     assert.equal(FX_DEFAULTS.cinematic, true);
     assert.equal(FX_DEFAULTS.glow, 0.5);   // 0.5 = 1.0x multiplier (stock)
 });
+
+
+test('bg styles: validated id set, particles default', () => {
+    const { BG_STYLE_IDS, readBgStyleSetting } = load().slopsmithViz_keys_highway_3d.__test;
+    // Host-realm copy — the vm array's foreign prototype trips deepEqual.
+    assert.deepEqual([...BG_STYLE_IDS], ['off', 'particles', 'lights', 'geometric']);
+    assert.equal(readBgStyleSetting(), 'particles');
+});
+
+test('FX defaults: ambience + score FX ship enabled', () => {
+    const { FX_DEFAULTS } = load().slopsmithViz_keys_highway_3d.__test;
+    assert.equal(FX_DEFAULTS.scoreFx, true);
+    assert.equal(FX_DEFAULTS.bgIntensity, 0.5);
+    assert.equal(FX_DEFAULTS.bgReactive, true);
+});