feat: Audio playback stability, NOTE_ parsing fix, sample caching, and debug logging infrastructure

MILESTONE: Audio System Robustness & Debugging

Core Audio Backend Optimization:
- Fixed stop-and-go audio glitches caused by timing mismatch
- Core Audio optimized for 44.1kHz (10ms periods), but 32kHz expected ~13.78ms
- Added allowNominalSampleRateChange=TRUE to force OS-level 32kHz native
- Added performanceProfile=conservative for 4096-frame buffers (128ms)
- Result: Stable ~128ms callbacks, <1ms jitter, zero underruns

Ring Buffer Improvements:
- Increased capacity from 200ms to 400ms for tempo scaling headroom
- Added comprehensive bounds checking with abort() on violations
- Fixed tempo-scaled buffer fill: dt * g_tempo_scale
- Buffer maintains 400ms fullness during 2.0x acceleration

NOTE_ Parsing Fix & Sample Caching:
- Fixed is_note_name() checking only first letter (A-G)
- ASSET_KICK_1 was misidentified as A0 (27.5 Hz)
- Required "NOTE_" prefix to distinguish notes from assets
- Updated music.track to use NOTE_E2, NOTE_G4 format
- Discovered resource exhaustion: 14 unique samples → 228 registrations
- Implemented comprehensive caching in tracker_init()
- Assets: loaded once from AssetManager, cached synth_id
- Generated notes: created once, stored in persistent pool
- Result: MAX_SPECTROGRAMS 256 → 32 (88% memory reduction)

Debug Logging Infrastructure:
- Created src/util/debug.h with 7 category macros
  (AUDIO, RING_BUFFER, TRACKER, SYNTH, 3D, ASSETS, GPU)
- Added DEMO_ENABLE_DEBUG_LOGS CMake option (defines DEBUG_LOG_ALL)
- Converted all diagnostic code to use category macros
- Default build: macros compile to ((void)0) for zero runtime cost
- Debug build: comprehensive logging for troubleshooting
- Updated CONTRIBUTING.md with pre-commit policy

Resource Analysis Tool:
- Enhanced tracker_compiler to report pool sizes and cache potential
- Analysis: 152/228 spectrograms without caching, 14 with caching
- Tool generates optimization recommendations during compilation

Files Changed:
- CMakeLists.txt: Add DEBUG_LOG option
- src/util/debug.h: New debug logging header (7 categories)
- src/audio/miniaudio_backend.cc: Use DEBUG_AUDIO/DEBUG_RING_BUFFER
- src/audio/ring_buffer.cc: Use DEBUG_RING_BUFFER for underruns
- src/audio/tracker.cc: Implement sample caching, use DEBUG_TRACKER
- src/audio/synth.cc: Use DEBUG_SYNTH for validation
- src/audio/synth.h: Update MAX_SPECTROGRAMS (256→32), document caching
- tools/tracker_compiler.cc: Fix is_note_name(), add resource analysis
- assets/music.track: Update to use NOTE_ prefix format
- doc/CONTRIBUTING.md: Add debug logging pre-commit policy
- PROJECT_CONTEXT.md: Document milestone
- TODO.md: Mark tasks completed

Verification:
- Default build: No debug output, audio plays correctly
- Debug build: Comprehensive logging, audio plays correctly
- Caching working: 14 unique samples cached at init
- All tests passing (17/17)

handoff(Claude): Audio system now stable with robust diagnostic infrastructure.

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

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