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, 8 insertions, 18 deletions

diff --git a/src/audio/audio.cc b/src/audio/audio.cc
index b00d416..67345cf 100644
--- a/src/audio/audio.cc
+++ b/src/audio/audio.cc
@@ -4,7 +4,7 @@
 
 #include "audio.h"
 #include "audio_backend.h"
-#include "miniaudio_backend.h"
+#include "backend/miniaudio_backend.h"
 #include "ring_buffer.h"
 #include "synth.h"
 #include "util/asset_manager.h"
@@ -40,23 +40,6 @@ AudioBackend* audio_get_backend() {
 }
 #endif /* !defined(STRIP_ALL) */
 
-int register_spec_asset(AssetId id) {
-  size_t size;
-  const uint8_t* data = GetAsset(id, &size);
-  if (!data || size < sizeof(SpecHeader))
-    return -1;
-
-  const SpecHeader* header = (const SpecHeader*)data;
-  const float* spectral_data = (const float*)(data + sizeof(SpecHeader));
-
-  Spectrogram spec;
-  spec.spectral_data_a = spectral_data;
-  spec.spectral_data_b = spectral_data; // No double-buffer for static assets
-  spec.num_frames = header->num_frames;
-
-  return synth_register_spectrogram(&spec);
-}
-
 void audio_init() {
   // Note: synth_init() must be called separately before using audio system.
   // In production code, use AudioEngine::init() which manages initialization
@@ -189,6 +172,13 @@ float audio_get_playback_time() {
          (RING_BUFFER_SAMPLE_RATE * RING_BUFFER_CHANNELS);
 }
 
+float audio_get_realtime_peak() {
+  if (g_audio_backend == nullptr) {
+    return 0.0f;
+  }
+  return g_audio_backend->get_realtime_peak();
+}
+
 // Expose ring buffer for backends
 AudioRingBuffer* audio_get_ring_buffer() {
   return &g_ring_buffer;