feat(keys_highway_3d): hit FX — vibrancy, timing-colored sparks, hit-line kick (K2) (#699)

- Note vibrancy: gem opacity 0.8 -> slider-driven (default 0.92),
  NOTE_EMISSIVE_BASE 0.08 -> 0.22, lane guides scale with the slider,
  _applyVibrancy retints the live scene without a chart rebuild
- Sparks (PORTED highway_3d, pool 96) at the struck key, colored by
  _timingHex/_classifyTiming (±100ms window, inner 40% = on-time; delta
  recovered from judgeHit's noteKey prefix — contract untouched);
  streak-scaled counts
- Hit-line brightness kick on scored presses (exp(-t*6) decay, hitFx
  slider), folded into the existing pulse incl. the bloom damp gate
- Settings: Hit sparks / Timing colours / Streak feedback + Hit feedback
  intensity + Note vibrancy sliders (keys3d_bg_*, live-applying)
- Tests: classifier boundaries, noteKey time round-trip, FX defaults
  (26 total)

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

plugins/keys_highway_3d/README.md

@@ -31,6 +31,12 @@ tunes one of these, mirror the change here (and in `drum_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 96 here — the flame sprites carry most of the hit feedback)
+- `_timingHex()` / `_classifyTiming()` — early/late/on-time feedback
+  colors (green/cyan/amber) + the 40%-window classifier
+- `_ssActive()` — host splitscreen probe (minus the guitar's focus-API
+  checks, which it needs for input routing and we don't)
 
 ## License