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, 50 insertions, 0 deletions

diff --git a/src/audio/backend/silent_backend.cc b/src/audio/backend/silent_backend.cc
new file mode 100644
index 0000000..637dd68
--- /dev/null
+++ b/src/audio/backend/silent_backend.cc
@@ -0,0 +1,50 @@
+// This file is part of the 64k demo project.
+// Implementation of silent backend for testing audio.cc.
+
+#include "silent_backend.h"
+
+#if !defined(STRIP_ALL)
+
+SilentBackend::SilentBackend()
+    : initialized_(false), started_(false), frames_rendered_(0),
+      voice_trigger_count_(0), test_peak_(0.0f) {
+}
+
+SilentBackend::~SilentBackend() {
+  shutdown();
+}
+
+void SilentBackend::init() {
+  initialized_ = true;
+}
+
+void SilentBackend::start() {
+  started_ = true;
+}
+
+void SilentBackend::shutdown() {
+  initialized_ = false;
+  started_ = false;
+}
+
+float SilentBackend::get_realtime_peak() {
+  // Return controllable test peak
+  return test_peak_;
+}
+
+void SilentBackend::on_voice_triggered(float timestamp, int spectrogram_id,
+                                        float volume, float pan) {
+  // Track voice triggers for testing
+  (void)timestamp;
+  (void)spectrogram_id;
+  (void)volume;
+  (void)pan;
+  voice_trigger_count_.fetch_add(1);
+}
+
+void SilentBackend::on_frames_rendered(int num_frames) {
+  // Track total frames rendered for testing
+  frames_rendered_.fetch_add(num_frames);
+}
+
+#endif /* !defined(STRIP_ALL) */