perf: stabilize audio engine for infinite-loop playback

[7se7en72025]

Summary

This PR resolves four compounding audio engine bottlenecks that resulted in
progressive crackling, main thread stalls, and eventual crashes during long or infinite Music Blocks sessions.

Changes

js/utils/synthutils.js

• POLYCOUNT 3 → 6: The polyphony limit of 3 voices was saturated
  instantly by two turtles playing chords, resulting in silent voice
  stealing and missing notes.
• _performNotes fast-path guard: paramsEffects is always a
  non-null object (always contains doPartials: true), so the existing
  paramsEffects === null fast-path was never taken. Every plain note
  re-wired the Web Audio graph via disconnect()/chain(). The fix
  checks whether any real effect nodes are needed; if not, in-place
  mutations are applied and triggerAttackRelease is called directly.
• +500 ms disposal buffer: Effect nodes were disposed at exactly
  beatValue * 1000 ms, coinciding with the note end. Any audio-clock
  jitter caused premature disposal (crackling). A 500 ms buffer absorbs drift.

js/turtle-singer.js

• Unhighlight timer deduplication: A new setTimeout for
  unhighlight(blk) was spawned on every note regardless of whether a
  previous one was still pending. In tight loops this accumulates
  thousands of stale timers and stalls the main thread. A
  _unhighlightTimers map now cancels the previous timer before
  setting a new one.## Testing
• No regressions found on basic note playback, effects (vibrato, chorus, tremolo), and drum patterns.
• Infinite "forever" loop with 4 turtles: did not crash after prolonged execution.

Criticality Scores (pre-fix)

Issue	Score
Rewiring audio graph per note	9/10
Polyphony exhaustion (POLYCOUNT=3)	8/10
GC thrashing / premature garbage collection	8/10
Timer queue flood (unhighlight)	7/10

[github-actions]

✅ All Jest tests passed! This PR is ready to merge.

[github-actions]

✅ All Jest tests passed! This PR is ready to merge.

[github-actions]

✅ All Jest tests passed! This PR is ready to merge.

[github-actions]

❌ Some Jest tests failed. Please check the logs and fix the issues before merging.

Failed Tests:

synthutils.test.js
turtle-singer.test.js

[github-actions]

✅ All Jest tests passed! This PR is ready to merge.

[7se7en72025]

Hi, @walterbender
this PR resolves audio crashes and buffer underruns for long/infinite playback:

synthutils.js: Increased POLYCOUNT from 3 to 6 to avoid voice starvation; introduced a fast path that bypasses audio graph rewiring for simple notes (the primary cause of underruns); introduced a 500 ms buffer for cleanup timers to avoid early synth garbage collection.
turtle-singer.js: Introduced timer deduplication (_unhighlightTimers) to cancel stale unhighlight timers in tight loops, preventing JS timer queue exhaustion. Together with upstream's MIN_HIGHLIGHT_DURATION_MS (400 ms), fast notes still yield a visible highlight.
All 117 test suites are passing. Rebased on the latest master.
Thanks for mainataining

[kartikktripathi]

@walterbender, approving. Changes are technically sound and improve stability under sustained/infinite playback without introducing regressions.
Thanks!