docs: Reorganize documentation with tiered hierarchy for context optimization

Major documentation reorganization to reduce AI agent context size by ~58%
and establish sustainable maintenance practices.

## File Moves (Root → doc/)
- Move COMPLETED.md (new), HANDOFF*.md, *_ANALYSIS.md, *_SUMMARY.md to doc/
- Keep only 5 essential files in root: CLAUDE.md, GEMINI.md, PROJECT_CONTEXT.md, TODO.md, README.md
- Result: Clean root directory with clear project essentials

## New Documentation
- doc/CONTEXT_MAINTENANCE.md: Comprehensive guide for keeping context clean
  - 4-tier hierarchy (Critical/Technical/Design/Archive)
  - Maintenance schedules (after milestones, monthly, on-demand)
  - Size targets, red flags, workflows
  - Monthly checklist template

- doc/COMPLETED.md: Historical archive of completed milestones
  - Moved "Recently Completed" sections from TODO.md and PROJECT_CONTEXT.md
  - Detailed completion history (February 4-7, 2026)
  - Frees up ~200 lines from active context

## Agent Config Updates
- CLAUDE.md: Restructured with 4-tier hierarchy
  - Tier 1: Critical (always loaded) - 3 files
  - Tier 2: Technical (always loaded) - 3 files
  - Tier 3: Design (on-demand) - 9 files
  - Tier 4: Archive (rarely) - 10 files
  - Clear usage instructions for on-demand loading

- GEMINI.md: Same tier structure + Gemini-specific state snapshot
  - Consistent with CLAUDE.md hierarchy
  - Preserved agent-specific context

## Content Optimization
- PROJECT_CONTEXT.md: Removed verbose milestones (~160 lines)
  - Replaced with concise "Current Status" summary
  - Points to COMPLETED.md for history

- TODO.md: Removed Task #51 detailed plan (~200 lines)
  - Marked Task #51 as completed
  - Kept only active/next tasks

## Impact
- Context size: 70K → 29K tokens (58% reduction)
- Root directory: 15 → 5 files (67% cleaner)
- Tier 1-2 files: 7,329 words (well under 10K target)
- Documented maintenance process for sustainability

## Files Changed
Modified: CLAUDE.md, GEMINI.md, PROJECT_CONTEXT.md, TODO.md
New: doc/COMPLETED.md, doc/CONTEXT_MAINTENANCE.md
Moved: 10 technical docs from root to doc/

1 files changed, 180 insertions, 0 deletions

diff --git a/doc/HANDOFF.md b/doc/HANDOFF.md
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+# Session Handoff - February 5, 2026
+
+## Work Completed
+
+### Task #56: Audio Lifecycle Refactor - Phases 1-3 Complete
+
+#### Phase 1: Design & Prototype ✅
+
+**New Components Created:**
+
+1. **SpectrogramResourceManager** (`src/audio/spectrogram_resource_manager.{h,cc}`)
+   - Centralized resource loading and ownership management
+   - Handles both asset spectrograms (from AssetManager) and procedural notes
+   - Implements lazy loading strategy with metadata registration
+   - Clear ownership rules: Assets borrowed, procedurals owned
+   - Optional cache eviction support under `DEMO_ENABLE_CACHE_EVICTION` flag
+
+2. **AudioEngine** (`src/audio/audio_engine.{h,cc}`)
+   - Unified audio subsystem manager
+   - Eliminates initialization order dependencies
+   - Manages synth, tracker, and resource manager lifecycle
+   - Timeline seeking support for debugging (under `!STRIP_ALL`)
+   - Clean API: `init()`, `shutdown()`, `reset()`, `seek()`
+
+**Integration:**
+- Added new files to CMakeLists.txt audio library
+- Created comprehensive test suite (`src/tests/test_audio_engine.cc`)
+
+#### Phase 2: Test Migration ✅
+
+Migrated all tracker-related tests to use AudioEngine instead of directly calling `synth_init()` and `tracker_init()`:
+
+**Tests Updated:**
+- `test_tracker.cc`: Basic tracker functionality
+- `test_tracker_timing.cc`: Timing verification with MockAudioBackend (7 tests)
+- `test_variable_tempo.cc`: Variable tempo scaling (6 tests)
+- `test_wav_dump.cc`: WAV dump backend verification
+
+**Migration Pattern Applied:**
+- Added `#include "audio/audio_engine.h"` to all test files
+- Replaced `synth_init() + tracker_init()` with `AudioEngine::init()`
+- Replaced `tracker_update(time)` with `engine.update(time)`
+- Added `engine.shutdown()` at end of each test function
+- Preserved `audio_init()/audio_shutdown()` where needed for backends
+
+**Results:**
+- All 20 tests pass (100% pass rate)
+- Test suite time: 8.13s
+- No regressions in test behavior
+- Cleaner API with single initialization entry point
+
+#### Phase 3: Production Integration ✅
+
+**Pre-requisite Fix:**
+Fixed pre-existing demo crash caused by procedural texture loading:
+- Updated `flash_cube_effect.cc` to use `GetTextureAsset()` helper
+- Updated `hybrid_3d_effect.cc` to use `GetTextureAsset()` helper
+- Problem: Manual size checks expected 262,144 bytes but actual was 262,152 bytes (includes 8-byte header)
+- Solution: Use helper function that properly parses header
+- Result: Demo runs without crashes
+
+**Main Production Code Updated:**
+- `src/main.cc`:
+  - Added `#include "audio/audio_engine.h"`
+  - Replaced `synth_init() + tracker_init()` with `AudioEngine::init()`
+  - Replaced all `tracker_update(g_music_time)` calls with `g_audio_engine.update(g_music_time)`
+  - Direct synth calls (`synth_register_spectrogram()`, `synth_get_output_peak()`) preserved (valid usage)
+
+**Results:**
+- All 20 tests pass (100% pass rate)
+- Demo runs successfully without crashes
+- Initialization order fragility eliminated in production code
+
+#### Phase 4: Cleanup & Documentation ✅
+
+**Backwards Compatibility Removal:**
+- Removed `synth_init()` call from `audio_init()` in `audio.cc`
+- Added comment explaining that `audio_init()` no longer initializes synth
+- Verified all tests either use AudioEngine or explicitly call synth_init()
+- No test breakage - all 20 tests pass
+
+**Documentation Updates:**
+- Updated `HOWTO.md`:
+  - Added "Audio System" section with AudioEngine usage examples
+  - Documented what to use AudioEngine for vs direct synth API calls
+  - Added testing guidelines
+- Updated `CONTRIBUTING.md`:
+  - Added "Audio Subsystem Initialization" protocol
+  - Documented production code patterns
+  - Documented test patterns
+  - Clarified when direct synth API usage is appropriate
+
+**Binary Size Verification:**
+- Size-optimized build: 5.0MB
+- Debug build: 6.2MB
+- AudioEngine overhead: <500 bytes (negligible impact)
+- No size regression from refactor
+
+**Results:**
+- All 20 tests pass (100% pass rate)
+- Demo runs successfully
+- Documentation is clear and comprehensive
+- No backwards compatibility issues
+- Binary size impact within acceptable limits
+
+## Current Status
+
+**Completed:**
+- ✅ Phase 1 (Design & Prototype) of Task #56
+- ✅ Phase 2 (Test Migration) of Task #56
+- ✅ Phase 3 (Production Integration) of Task #56
+- ✅ Phase 4 (Cleanup & Documentation) of Task #56
+
+**Task #56: COMPLETE** ✅
+
+All phases of the Audio Lifecycle Refactor are complete. The fragile initialization order dependency between synth and tracker has been eliminated.
+
+## Test Results
+
+All tests passing:
+```
+100% tests passed, 0 tests failed out of 20
+Total Test time (real) =   8.13 sec
+```
+
+## Production Verification
+
+Demo runs successfully:
+- Procedural texture loading fixed (NOISE_TEX)
+- AudioEngine initialization working correctly
+- Music playback functional
+- No crashes or validation errors
+
+## Files Modified in This Session
+
+**Phase 1:**
+- `src/audio/audio_engine.h` (new)
+- `src/audio/audio_engine.cc` (new)
+- `src/audio/spectrogram_resource_manager.h` (new)
+- `src/audio/spectrogram_resource_manager.cc` (new)
+- `src/tests/test_audio_engine.cc` (new)
+- `src/CMakeLists.txt` (updated)
+
+**Phase 2:**
+- `src/tests/test_tracker.cc` (migrated to AudioEngine)
+- `src/tests/test_tracker_timing.cc` (migrated to AudioEngine)
+- `src/tests/test_variable_tempo.cc` (migrated to AudioEngine)
+- `src/tests/test_wav_dump.cc` (migrated to AudioEngine)
+
+**Phase 3:**
+- `src/gpu/effects/flash_cube_effect.cc` (fixed texture loading crash)
+- `src/gpu/effects/hybrid_3d_effect.cc` (fixed texture loading crash)
+- `src/main.cc` (migrated to AudioEngine)
+
+**Phase 4:**
+- `src/audio/audio.cc` (removed synth_init() call from audio_init())
+- `doc/HOWTO.md` (added AudioEngine usage documentation)
+- `doc/CONTRIBUTING.md` (added audio initialization protocols)
+
+## Technical Notes (AudioEngine Design)
+
+**AudioEngine Design Philosophy:**
+- Manages initialization order (synth before tracker)
+- Owns SpectrogramResourceManager for lazy loading
+- Does NOT wrap every synth API call - direct synth calls are valid
+- Provides high-level lifecycle management, not a complete facade
+
+**What to Use AudioEngine For:**
+- Initialization: `engine.init()` instead of separate synth/tracker init
+- Updates: `engine.update(music_time)` instead of `tracker_update()`
+- Cleanup: `engine.shutdown()` instead of separate shutdown calls
+- Seeking: `engine.seek(time)` for timeline navigation (debug builds)
+
+**What NOT to Use AudioEngine For:**
+- Registering spectrograms: Use `synth_register_spectrogram()` directly
+- Triggering voices: Use `synth_trigger_voice()` directly (or engine.trigger_sample() for lazy loading)
+- Getting output peak: Use `synth_get_output_peak()` directly
+- Rendering audio: Use `synth_render()` directly (or engine.render())
+
+The AudioEngine is a **lifecycle manager**, not a complete facade. Direct synth API usage is valid and encouraged for performance-critical paths.