docs: Milestone state snapshot - Audio system robustness complete

Documented completion of major audio system improvements:
- Task #51: Tracker timing verification (0.000ms sync confirmed)
- Variable tempo system (6 tests, all passing)
- Zero production size impact (all test code under !STRIP_ALL)

Key achievements:
- Perfect audio synchronization verified
- Dynamic tempo control ready (music_time abstraction)
- Comprehensive test coverage (16/16 tests passing)
- Mathematical precision verified for reset tricks

Updated:
- PROJECT_CONTEXT.md: Added milestone section
- HANDOFF_2026-02-04.md: Complete state snapshot for handoff

System ready for:
- Dynamic tempo animation
- Physics & collision (Task #49)
- 3D scene pipeline (Task #18)

handoff(Claude): Audio milestone complete, state saved

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

1 files changed, 219 insertions, 0 deletions

diff --git a/HANDOFF_2026-02-04.md b/HANDOFF_2026-02-04.md
new file mode 100644
index 0000000..f38e5be
--- /dev/null
+++ b/HANDOFF_2026-02-04.md
@@ -0,0 +1,219 @@
+# Handoff: Audio System Milestone (February 4, 2026)
+
+## Summary
+Completed comprehensive audio system robustness improvements including tracker timing verification and variable tempo implementation.
+
+## What Was Accomplished
+
+### Task #51: Tracker Timing Verification System
+**Goal**: Create robust testing infrastructure to verify audio synchronization.
+
+**Implementation**:
+- Created `AudioBackend` interface (`src/audio/audio_backend.h`) separating synthesis from output
+- Implemented `MiniaudioBackend` for production use
+- Implemented `MockAudioBackend` for testing with event recording
+- Added time tracking to `synth.cc` (32kHz sample rate precision)
+- Created comprehensive test suite (`test_tracker_timing.cc`)
+
+**Key Finding**: ✅ Simultaneous pattern triggers have **0.000ms delta** (perfect sync verified)
+
+**Files**:
+- `src/audio/audio_backend.h` (interface)
+- `src/audio/miniaudio_backend.{h,cc}` (production backend)
+- `src/audio/mock_audio_backend.{h,cc}` (test backend, under `!STRIP_ALL`)
+- `src/audio/audio.cc` (refactored to use backends)
+- `src/audio/synth.cc` (added time tracking hooks)
+- `src/tests/test_audio_backend.cc` (3 tests)
+- `src/tests/test_mock_backend.cc` (6 tests)
+- `src/tests/test_tracker_timing.cc` (7 tests)
+
+**Test Results**: 13 → 16 tests passing (100% success rate)
+
+### Variable Tempo System
+**Goal**: Enable dynamic tempo changes without pitch shifting or BPM dependencies.
+
+**Design Philosophy**:
+- Don't modify spectrograms or synthesis engine
+- Only change WHEN patterns trigger (not HOW they sound)
+- Simple: `music_time += dt * tempo_scale`
+
+**Implementation**:
+- Added `g_music_time`, `g_tempo_scale`, `g_last_physical_time` to `main.cc`
+- Modified `update_game_logic` to calculate delta time and advance music_time
+- Changed `tracker_update((float)t)` → `tracker_update(g_music_time)`
+- Created test suite (`test_variable_tempo.cc`) with 6 scenarios
+
+**Test Coverage**:
+1. Basic tempo scaling (1.0x, 2.0x, 0.5x) ✅
+2. 2x speed-up reset trick (accelerate to 2.0x, reset to 1.0x) ✅
+3. 2x slow-down reset trick (decelerate to 0.5x, reset to 1.0x) ✅
+4. Pattern density swap at reset points ✅
+5. Continuous acceleration (0.5x to 2.0x over 10s) ✅
+6. Oscillating tempo (sine wave modulation) ✅
+
+**Key Results**:
+- After 5s physical at 2.0x tempo: `music_time=7.550s` (expected ~7.5s) ✅
+- Reset to 1.0x, advance 2s: `music_time delta=2.000s` (expected ~2.0s) ✅
+- All mathematical predictions verified with <0.1s tolerance
+
+**Files**:
+- `src/main.cc` (~15 lines added)
+- `src/tests/test_variable_tempo.cc` (376 lines, 6 tests)
+- `ANALYSIS_VARIABLE_TEMPO.md` (original complex approach)
+- `ANALYSIS_VARIABLE_TEMPO_V2.md` (simplified approach - implemented)
+
+## Current State
+
+### Build Status
+- ✅ All 16 tests passing (100% success rate)
+- ✅ Clean build with no warnings
+- ✅ Zero size impact on stripped builds (`STRIP_ALL`)
+
+### Audio System
+- ✅ Backend abstraction layer working
+- ✅ Mock backend for deterministic testing
+- ✅ Perfect audio synchronization verified (0.000ms delta)
+- ✅ Variable tempo ready for dynamic control
+- ✅ No pitch shifting artifacts
+
+### Ready for Next Steps
+The audio system is now robust and ready for:
+- Dynamic tempo control during playback (animate `g_tempo_scale`)
+- "Reset trick" implementation (accelerate/slow + pattern density swap)
+- Tempo curves (linear, exponential, oscillating, manual)
+- Integration with visual sync and beat detection
+
+## Technical Details
+
+### Music Time Formula
+```cpp
+// Physical time → Music time
+float dt = current_time - last_time;
+music_time += dt * tempo_scale;
+
+// Examples:
+// tempo_scale=1.0 → music_time advances at normal rate
+// tempo_scale=2.0 → music_time advances 2x faster (patterns trigger sooner)
+// tempo_scale=0.5 → music_time advances 2x slower (patterns trigger later)
+```
+
+### Reset Trick Pattern
+```cpp
+// Phase 1: Accelerate
+tempo_scale = 1.0f + (time / 5.0f);  // Linear acceleration
+if (tempo_scale >= 2.0f) tempo_scale = 2.0f;
+
+// Phase 2: Reset
+if (tempo_scale >= 2.0f) {
+    tempo_scale = 1.0f;  // Reset to normal
+    // Switch to 2x denser pattern to maintain perceived tempo
+}
+```
+
+### MockAudioBackend API
+```cpp
+// Record events
+void on_voice_triggered(float timestamp, int spectrogram_id,
+                        float volume, float pan) override;
+
+// Time management
+void on_frames_rendered(int num_frames) override;
+float get_current_time() const;
+
+// Test queries
+const std::vector<VoiceTriggerEvent>& get_events() const;
+void clear_events();
+```
+
+## Commits Made
+
+1. `c4888be` - feat(audio): Audio backend abstraction layer (Task #51.1)
+2. `bb49daa` - feat(audio): Mock audio backend for testing (Task #51.2)
+3. `215a4d8` - feat(audio): Tracker timing test suite (Tasks #51.3 & #51.4)
+4. `a09e36b` - docs: Variable tempo architecture analysis
+5. `829e211` - docs: Simplified variable tempo approach (V2)
+6. `25d7e4e` - feat(audio): Variable tempo system with music time abstraction
+
+## Documentation Updated
+- ✅ `TODO.md` - Marked Task #51 complete, added variable tempo section
+- ✅ `PROJECT_CONTEXT.md` - Added milestone to "Recently Completed"
+- ✅ `ANALYSIS_VARIABLE_TEMPO.md` - Original complex analysis
+- ✅ `ANALYSIS_VARIABLE_TEMPO_V2.md` - Simplified approach (implemented)
+- ✅ `HANDOFF_2026-02-04.md` - This document
+
+## Next Steps (Recommendations)
+
+### Immediate (Optional)
+- Add tempo control API (`set_tempo_scale()`, `get_tempo_scale()`, `get_music_time()`)
+- Expose tempo control to demo sequence system
+- Test with actual demo playback
+
+### Priority 1: Physics & Collision (Task #49)
+- Implement CPU-side SDF library
+- Build BVH for acceleration
+- Create physics loop with collision resolution
+
+### Priority 2: 3D System Enhancements (Task #18)
+- Blender exporter script
+- Asset ingestion for 3D scenes
+- Runtime scene loader
+
+## Notes for Next Developer
+
+### Testing Strategy
+- All audio tests use `MockAudioBackend` for deterministic verification
+- Use `audio_set_backend(&mock)` before tests
+- Check `get_events()` for voice trigger timing
+- Compare timestamps with expected values (<1ms tolerance)
+
+### Variable Tempo Usage
+```cpp
+// In main loop or sequence system
+extern float g_tempo_scale;  // Declared in main.cc
+g_tempo_scale = 1.5f;  // 1.5x faster playback
+
+// For gradual acceleration
+g_tempo_scale = 1.0f + (time / 10.0f);  // Accelerate over 10s
+
+// For reset trick
+if (g_tempo_scale >= 2.0f) {
+    g_tempo_scale = 1.0f;
+    trigger_denser_pattern();
+}
+```
+
+### Code Philosophy
+- Keep spectrograms unchanged (size optimization)
+- All timing in "music time" space
+- Physical time only for delta calculation
+- Zero production overhead (`!STRIP_ALL` guards)
+
+## Verification Commands
+
+```bash
+# Build and test
+cmake --build build
+cd build && ctest
+
+# Coverage report
+./scripts/gen_coverage_report.sh src/audio
+
+# Size check (stripped build)
+cmake -S . -B build_strip -DDEMO_STRIP_ALL=ON
+cmake --build build_strip
+ls -lh build_strip/demo64k*
+```
+
+## Status: MILESTONE COMPLETE ✅
+
+All objectives achieved:
+- ✅ Audio synchronization verified (0.000ms delta)
+- ✅ Variable tempo system implemented and tested
+- ✅ Zero production size impact
+- ✅ 100% test pass rate
+- ✅ Documentation complete
+- ✅ Ready for next phase
+
+---
+*Generated: February 4, 2026*
+*Claude Sonnet 4.5*