fix(audio): Remove sample offsets - incompatible with tempo scaling

This fixes the irregular timing caused by mixing music time and physical time.

ROOT CAUSE (THE REAL BUG):
Sample offset calculation was mixing two incompatible time domains:
1. event_trigger_time: in MUSIC TIME (tempo-scaled, can be 2x faster)
2. current_render_time: in PHYSICAL TIME (1:1 with real time, not scaled)

When tempo != 1.0, these diverge dramatically:
  Example at 2.0x tempo:
    - Music time: 10.0s (advanced 2x faster)
    - Physical render time: 5.0s (real time elapsed)
    - Calculated offset: (10.0 - 5.0) * 32000 = 160000 samples = 5 SECONDS!
    - Result: Event triggers 5 seconds late

This caused irregular timing because:
- At tempo 1.0x: offsets were roughly correct (domains aligned)
- At tempo != 1.0x: offsets were wildly wrong (domains diverged)
- Result: Random jitter as tempo changed

WHY WAV DUMP WORKED:
WAV dump doesn't use tempo scaling (g_tempo_scale = 1.0), so music_time ≈
physical_time and the domains stayed aligned by accident.

THE SOLUTION:
Remove sample offsets entirely. Trigger events immediately when music_time
passes their trigger time. Accept ~16ms quantization (one frame at 60fps).

TRADE-OFFS:
- Before: Attempted sample-accurate timing (but broken with tempo scaling)
- After: ~16ms quantization (acceptable for rhythmic events)
- Benefit: Consistent timing across all tempo values
- Benefit: Same behavior in WAV dump and miniaudio playback

CHANGES:
- tracker.cc: Remove offset calculation, always pass offset=0
- Simplify event triggering logic
- Add comment explaining why offsets don't work with tempo scaling

Previous commits (9cae6f1, 7271773) attempted to fix this with render_time
tracking, but missed the fundamental issue: you can't calculate sample offsets
when event times and render times are in different time domains.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

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