feat(audio): Complete Phase 4 - Cleanup and Documentation (Task #56)

Completed final cleanup phase of Audio Lifecycle Refactor. Removed backwards
compatibility shims and updated documentation to reflect new AudioEngine-based
initialization patterns.

Changes:

1. Removed Backwards Compatibility:
   - Removed synth_init() call from audio_init() in audio.cc
   - Added comment explaining AudioEngine is the preferred initialization method
   - All tests already explicitly call synth_init() or use AudioEngine

2. Documentation Updates:
   - Updated HOWTO.md with AudioEngine usage examples and best practices
   - Updated CONTRIBUTING.md with audio subsystem initialization protocols
   - Documented when to use AudioEngine vs direct synth API calls
   - Clarified that AudioEngine is a lifecycle manager, not a complete facade

3. Size Verification:
   - Size-optimized build: 5.0MB (vs 6.2MB debug)
   - AudioEngine overhead: <500 bytes (within acceptable limits)
   - No size regression from refactor

Results:
- All 20 tests pass (100% pass rate)
- Demo runs successfully
- No backwards compatibility issues
- Clear documentation for future development
- Binary size impact negligible

Design Philosophy:
- AudioEngine manages initialization order (synth before tracker)
- Direct synth API calls remain valid for performance-critical paths
- Low-level tests can still use synth_init() directly if needed
- Preferred pattern: Use AudioEngine for lifecycle, direct APIs for operations

handoff(Claude): Completed Task #56 Phase 4 - All phases complete! Audio
Lifecycle Refactor is fully implemented, tested, and documented. The
fragile initialization order dependency has been eliminated.

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

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