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, 65 insertions, 0 deletions

diff --git a/src/audio/backend/mock_audio_backend.h b/src/audio/backend/mock_audio_backend.h
new file mode 100644
index 0000000..55e89c5
--- /dev/null
+++ b/src/audio/backend/mock_audio_backend.h
@@ -0,0 +1,65 @@
+// This file is part of the 64k demo project.
+// It implements a test-only mock audio backend for event recording.
+// Used for tracker timing verification and audio synchronization tests.
+
+#pragma once
+
+#if !defined(STRIP_ALL)
+
+#include "../audio_backend.h"
+#include <vector>
+
+// Event structure for recorded voice triggers
+struct VoiceTriggerEvent {
+  float timestamp_sec;
+  int spectrogram_id;
+  float volume;
+  float pan;
+};
+
+// Mock audio backend that records all voice trigger events
+// Used for testing tracker timing and synchronization
+class MockAudioBackend : public AudioBackend {
+ public:
+  MockAudioBackend();
+  ~MockAudioBackend() override;
+
+  // AudioBackend interface (no-ops for mock)
+  void init() override;
+  void start() override;
+  void shutdown() override;
+  float get_realtime_peak() override;
+
+  // Event recording hooks
+  void on_voice_triggered(float timestamp, int spectrogram_id, float volume,
+                          float pan) override;
+  void on_frames_rendered(int num_frames) override;
+
+  // Test interface methods
+  const std::vector<VoiceTriggerEvent>& get_events() const {
+    return recorded_events_;
+  }
+  void clear_events() {
+    recorded_events_.clear();
+  }
+
+  // Manual time control for deterministic testing
+  void advance_time(float delta_sec) {
+    current_time_sec_ += delta_sec;
+  }
+  void set_time(float time_sec) {
+    current_time_sec_ = time_sec;
+  }
+  float get_current_time() const {
+    return current_time_sec_;
+  }
+
+  // Sample rate used for frame-to-time conversion
+  static const int kSampleRate = 32000;
+
+ private:
+  std::vector<VoiceTriggerEvent> recorded_events_;
+  float current_time_sec_;
+};
+
+#endif /* !defined(STRIP_ALL) */