feat(audio): Implement AudioEngine and SpectrogramResourceManager (Task #56 Phase 1)

Implements Phase 1 of the audio lifecycle refactor to eliminate initialization
order dependencies between synth and tracker.

New Components:

1. SpectrogramResourceManager (src/audio/spectrogram_resource_manager.{h,cc})
   - Centralized resource loading and ownership
   - Lazy loading: resources registered but not loaded until needed
   - Handles both asset spectrograms and procedural notes
   - Clear ownership: assets borrowed, procedurals owned
   - Optional cache eviction under DEMO_ENABLE_CACHE_EVICTION flag

2. AudioEngine (src/audio/audio_engine.{h,cc})
   - Unified audio subsystem manager
   - Single initialization point eliminates order dependencies
   - Manages synth, tracker, and resource manager lifecycle
   - Timeline seeking API for debugging (!STRIP_ALL)
   - Clean API: init(), shutdown(), reset(), seek()

Features:
- Lazy loading strategy with manual preload API
- Reset functionality for timeline seeking
- Zero impact on production builds
- Debug-only seeking support

Testing:
- Comprehensive test suite (test_audio_engine.cc)
- Tests lifecycle, resource loading, reset, seeking
- All 20 tests passing (100% pass rate)

Bug Fixes:
- Fixed infinite recursion in AudioEngine::tracker_reset()

Integration:
- Added to CMakeLists.txt audio library
- No changes to existing code (backward compatible)

Binary Size Impact: ~700 bytes (within budget)

Next: Phase 2 (Test Migration) - Update existing tests to use AudioEngine

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

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