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/wav_dump_backend.h b/src/audio/backend/wav_dump_backend.h
new file mode 100644
index 0000000..de445d6
--- /dev/null
+++ b/src/audio/backend/wav_dump_backend.h
@@ -0,0 +1,65 @@
+// This file is part of the 64k demo project.
+// WAV dump backend for debugging audio output to file instead of device.
+// Stripped in final build (STRIP_ALL).
+
+#if !defined(DEMO_AUDIO_WAV_DUMP_BACKEND_H)
+#define DEMO_AUDIO_WAV_DUMP_BACKEND_H
+
+#include "../audio_backend.h"
+#include <stdio.h>
+#include <vector>
+
+#if !defined(STRIP_ALL)
+
+// WAV file dump backend for debugging
+// Captures audio from synth_render() and writes to .wav file
+class WavDumpBackend : public AudioBackend {
+ public:
+  WavDumpBackend();
+  ~WavDumpBackend();
+
+  // AudioBackend interface
+  void init() override;
+  void start() override;
+  void shutdown() override;
+  float get_realtime_peak() override;
+
+  // Set output filename (call before init())
+  void set_output_file(const char* filename);
+
+  // Write audio data to WAV file (stereo interleaved float samples)
+  // num_samples: Total number of samples (2x num_frames for stereo)
+  void write_audio(const float* samples, int num_samples);
+
+  // Get total samples written
+  size_t get_samples_written() const {
+    return samples_written_;
+  }
+
+  // Get number of samples that were clipped (diagnostic metric)
+  size_t get_clipped_samples() const {
+    return clipped_samples_;
+  }
+
+ private:
+  // Write WAV header with known sample count
+  void write_wav_header(FILE* file, uint32_t num_samples);
+
+  // Update WAV header with final sample count
+  void update_wav_header();
+
+  FILE* wav_file_;
+  std::vector<float> sample_buffer_;
+  size_t samples_written_;
+  size_t clipped_samples_;
+  const char* output_filename_;
+  bool is_active_;
+  float duration_sec_;
+
+  static const int kSampleRate = 32000;
+  static const int kBufferSize = 1024;
+};
+
+#endif /* !defined(STRIP_ALL) */
+
+#endif /* DEMO_AUDIO_WAV_DUMP_BACKEND_H */