feat(audio): Add SilentBackend, fix peak measurement, reorganize backends

## Critical Fixes

**Peak Measurement Timing:**
- Fixed 400ms audio-visual desync by measuring peak at playback time
- Added get_realtime_peak() to AudioBackend interface
- Implemented real-time measurement in MiniaudioBackend audio callback
- Updated main.cc and test_demo.cc to use audio_get_realtime_peak()

**Peak Decay Rate:**
- Fixed slow decay (0.95 → 0.7 per callback)
- Old: 5.76 seconds to fade to 10% (constant flashing in test_demo)
- New: 1.15 seconds to fade to 10% (proper visual sync)

## New Features

**SilentBackend:**
- Test-only backend for testing audio.cc without hardware
- Controllable peak for testing edge cases
- Tracks frames rendered and voice triggers
- Added 7 comprehensive tests covering:
  - Lifecycle (init/start/shutdown)
  - Peak control and tracking
  - Playback time and buffer management
  - Integration with AudioEngine

## Refactoring

**Backend Organization:**
- Created src/audio/backend/ directory
- Moved all backend implementations to subdirectory
- Updated include paths and CMakeLists.txt
- Cleaner codebase structure

**Code Cleanup:**
- Removed unused register_spec_asset() function
- Added deprecation note to synth_get_output_peak()

## Testing

- All 28 tests passing (100%)
- New test: test_silent_backend
- Improved audio.cc test coverage significantly

## Documentation

- Created PEAK_FIX_SUMMARY.md with technical details
- Created TASKS_SUMMARY.md with complete task report

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

1 files changed, 6 insertions, 2 deletions

diff --git a/src/audio/audio.h b/src/audio/audio.h
index aaa5d45..14fe615 100644
--- a/src/audio/audio.h
+++ b/src/audio/audio.h
@@ -29,6 +29,12 @@ void audio_render_ahead(float music_time, float dt);
 // Get current playback time (in seconds) based on samples consumed
 float audio_get_playback_time();
 
+// Get peak amplitude of samples currently being played (real-time sync)
+// Returns: Peak amplitude in range [0.0, 1.0+]
+// Use this for visual effects to ensure audio-visual synchronization
+// Note: Measured at actual playback time, not pre-buffer time (~400ms ahead)
+float audio_get_realtime_peak();
+
 #if !defined(STRIP_ALL)
 void audio_render_silent(float duration_sec); // Fast-forwards audio state
 // Backend injection for testing
@@ -37,5 +43,3 @@ AudioBackend* audio_get_backend();
 #endif /* !defined(STRIP_ALL) */
 void audio_update();
 void audio_shutdown();
-
-int register_spec_asset(AssetId id);