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, 52 insertions, 0 deletions

diff --git a/src/audio/backend/silent_backend.h b/src/audio/backend/silent_backend.h
new file mode 100644
index 0000000..f7da42d
--- /dev/null
+++ b/src/audio/backend/silent_backend.h
@@ -0,0 +1,52 @@
+// This file is part of the 64k demo project.
+// It implements a silent test backend for testing audio.cc without hardware.
+// Useful for achieving high test coverage and triggering edge cases.
+
+#pragma once
+
+#if !defined(STRIP_ALL)
+
+#include "../audio_backend.h"
+#include <atomic>
+
+// Silent backend for testing - no audio output, pure inspection
+// Allows testing audio.cc logic (buffer management, playback time tracking)
+// without requiring audio hardware or miniaudio
+class SilentBackend : public AudioBackend {
+ public:
+  SilentBackend();
+  ~SilentBackend() override;
+
+  // AudioBackend interface
+  void init() override;
+  void start() override;
+  void shutdown() override;
+  float get_realtime_peak() override;
+
+  // Test inspection interface
+  bool is_initialized() const { return initialized_; }
+  bool is_started() const { return started_; }
+  int get_frames_rendered() const { return frames_rendered_.load(); }
+  int get_voice_trigger_count() const { return voice_trigger_count_.load(); }
+
+  // Manual control for testing edge cases
+  void set_peak(float peak) { test_peak_ = peak; }
+  void reset_stats() {
+    frames_rendered_ = 0;
+    voice_trigger_count_ = 0;
+  }
+
+  // Event hooks (inherited from AudioBackend)
+  void on_voice_triggered(float timestamp, int spectrogram_id, float volume,
+                          float pan) override;
+  void on_frames_rendered(int num_frames) override;
+
+ private:
+  bool initialized_;
+  bool started_;
+  std::atomic<int> frames_rendered_;
+  std::atomic<int> voice_trigger_count_;
+  float test_peak_; // Controllable peak for testing
+};
+
+#endif /* !defined(STRIP_ALL) */