feat(audio): Complete Phase 2 - Migrate tests to AudioEngine (Task #56)

Migrated all tracker-related tests to use AudioEngine instead of directly
calling synth_init() and tracker_init(), eliminating fragile initialization
order dependencies.

Tests Migrated:
- 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:
- Added AudioEngine include 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 (slightly faster)
- No regressions in test behavior
- Cleaner API with single initialization entry point

Next Steps (Phase 3):
- Migrate main.cc and production code to use AudioEngine
- Add backwards compatibility shims during transition

handoff(Claude): Completed Task #56 Phase 2 - all tracker tests now use
AudioEngine. The initialization order fragility is eliminated in test code.
Ready for Phase 3 (production integration).

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