refactor(audio): Remove tempo logic from WavDumpBackend

Fixed design flaw where WavDumpBackend had hardcoded tempo curves
duplicating logic from main.cc. Backend should be passive and just
write audio data, not implement simulation logic.

Changes:
- WavDumpBackend.start() is now non-blocking (was blocking simulation loop)
- Added write_audio() method for passive audio writing
- Removed all tempo scaling logic from backend (lines 62-97)
- Removed tracker_update() and audio_render_ahead() calls from backend
- Removed set_duration() (no longer needed, frontend controls duration)

Frontend (main.cc):
- Added WAV dump mode loop that drives simulation with its own tempo logic
- Reads from ring buffer and calls wav_backend.write_audio()
- Tempo logic stays in one place (no duplication)
- Added ring_buffer.h include for AudioRingBuffer access

Test (test_wav_dump.cc):
- Updated to use frontend-driven approach
- Test manually drives simulation loop
- Calls write_audio() after each frame
- Verifies passive backend behavior

Design:
- Backend: Passive file writer (init/start/write_audio/shutdown)
- Frontend: Active simulation driver (tempo, tracker, rendering)
- Zero duplication of tempo/simulation logic
- Clean separation of concerns

All 27 tests pass.

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

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