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, 51 insertions, 0 deletions

diff --git a/src/audio/backend/mock_audio_backend.cc b/src/audio/backend/mock_audio_backend.cc
new file mode 100644
index 0000000..068d8a3
--- /dev/null
+++ b/src/audio/backend/mock_audio_backend.cc
@@ -0,0 +1,51 @@
+// This file is part of the 64k demo project.
+// It implements the mock audio backend for testing.
+// Records voice trigger events with precise timestamps.
+
+#include "mock_audio_backend.h"
+
+#if !defined(STRIP_ALL)
+
+MockAudioBackend::MockAudioBackend() : current_time_sec_(0.0f) {
+}
+
+MockAudioBackend::~MockAudioBackend() {
+}
+
+void MockAudioBackend::init() {
+  // No-op for mock backend
+}
+
+void MockAudioBackend::start() {
+  // No-op for mock backend
+}
+
+void MockAudioBackend::shutdown() {
+  // No-op for mock backend
+}
+
+float MockAudioBackend::get_realtime_peak() {
+  // Mock backend: Return synthetic peak for testing
+  // Can be enhanced later to track actual synth output if needed
+  return 0.5f;
+}
+
+void MockAudioBackend::on_voice_triggered(float timestamp, int spectrogram_id,
+                                          float volume, float pan) {
+  // Record the event with the timestamp provided by synth
+  VoiceTriggerEvent event;
+  event.timestamp_sec = timestamp;
+  event.spectrogram_id = spectrogram_id;
+  event.volume = volume;
+  event.pan = pan;
+  recorded_events_.push_back(event);
+}
+
+void MockAudioBackend::on_frames_rendered(int num_frames) {
+  // Update internal time based on frames rendered
+  // This is called by audio_render_silent() for seek/fast-forward simulation
+  const float delta_sec = (float)num_frames / (float)kSampleRate;
+  current_time_sec_ += delta_sec;
+}
+
+#endif /* !defined(STRIP_ALL) */