feat(drum_highway_3d): hit FX — sparks, timing colors, lane flashes, kick pulse, open hi-hat (D2) (#697)

- Pooled spark bursts (PORTED highway_3d, pool 160) at the struck lane,
  timing-colored via _timingHex/_classifyTiming (±50ms window, inner 40%
  = on-time); streak-scaled counts (streakFx)
- Lane flashes resurrected as pooled additive gauss-tex quads at the hit
  line (timing-colored; red for wrong hits); kick = full-width quad
- Kick pulse: camera dip + amber floor wash, exp decay, hitFx-scaled
- Approach highlight: lane stripes brighten toward their next note
- Open hi-hat: hh_open renders a warm ring around the gem (closes TODO);
  orthogonal to accent/ghost/flam variants
- Settings: Hit sparks / Timing colours / Streak feedback toggles +
  Hit feedback intensity slider (drum_h3d_bg_*, live-applying)
- All FX resources pooled/shared, disposed in BOTH teardown paths
- Tests: _classifyTiming boundaries + FX defaults (10 total)

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

plugins/drum_highway_3d/README.md

@@ -71,6 +71,13 @@ tunes one of these, mirror the change here (and in `keys_highway_3d`):
   mapping switch in `draw()`; addons dynamic-imported from
   `/static/vendor/three/addons/` (no CDN fallback — direct render is the
   graceful degrade)
+- `_sparkBurst()` / `_sparkUpdate()` — pooled additive Points hit sparks
+  (pool 160 here vs the guitar's 256)
+- `_makeGaussTex()` — soft-falloff DataTexture for the additive lane-flash
+  quads
+- `_timingHex()` — early/late/on-time feedback colors (green/cyan/amber)
+- `_ssActive()` — host splitscreen probe (minus the guitar's focus-API
+  checks, which it needs for input routing and we don't)
 
 ## Why a separate plugin (vs. drum mode inside highway_3d)?