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, 60 insertions, 0 deletions

diff --git a/src/audio/backend/jittered_audio_backend.h b/src/audio/backend/jittered_audio_backend.h
new file mode 100644
index 0000000..8bda4c7
--- /dev/null
+++ b/src/audio/backend/jittered_audio_backend.h
@@ -0,0 +1,60 @@
+// This file is part of the 64k demo project.
+// It implements a test backend that simulates jittered audio consumption.
+// Useful for stress-testing ring buffer under realistic timing conditions.
+
+#pragma once
+
+#if !defined(STRIP_ALL)
+
+#include "../audio_backend.h"
+#include <atomic>
+#include <thread>
+
+// Simulates a real audio device with timing jitter and variable chunk sizes
+class JitteredAudioBackend : public AudioBackend {
+ public:
+  JitteredAudioBackend();
+  ~JitteredAudioBackend() override;
+
+  void init() override;
+  void start() override;
+  void shutdown() override;
+  float get_realtime_peak() override;
+
+  // Control simulation
+  void set_jitter_amount(float jitter_ms); // Random jitter in ms (default: 5ms)
+  void set_base_interval(
+      float interval_ms); // Base interval between reads (default: 10ms)
+  void set_chunk_size_range(int min_frames,
+                            int max_frames); // Variable chunk sizes
+
+  // Query state
+  int get_total_frames_consumed() const {
+    return total_frames_consumed_.load();
+  }
+  int get_underrun_count() const {
+    return underrun_count_.load();
+  }
+  bool is_running() const {
+    return running_.load();
+  }
+
+ private:
+  void audio_thread_loop();
+
+  std::thread audio_thread_;
+  std::atomic<bool> running_;
+  std::atomic<bool> should_stop_;
+
+  // Configuration
+  float jitter_ms_;
+  float base_interval_ms_;
+  int min_chunk_frames_;
+  int max_chunk_frames_;
+
+  // Statistics
+  std::atomic<int> total_frames_consumed_;
+  std::atomic<int> underrun_count_; // How many times buffer was empty
+};
+
+#endif /* !defined(STRIP_ALL) */