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, 1 insertions, 12 deletions

diff --git a/src/tests/test_jittered_audio.cc b/src/tests/test_jittered_audio.cc
index f880c74..8afb8c0 100644
--- a/src/tests/test_jittered_audio.cc
+++ b/src/tests/test_jittered_audio.cc
@@ -6,7 +6,7 @@
 #if !defined(STRIP_ALL)
 
 #include "audio/audio.h"
-#include "audio/jittered_audio_backend.h"
+#include "audio/backend/jittered_audio_backend.h"
 #include "audio/synth.h"
 #include "audio/tracker.h"
 #include <assert.h>
@@ -118,17 +118,6 @@ void test_jittered_audio_with_acceleration() {
 
     // Sleep to simulate frame time
     std::this_thread::sleep_for(std::chrono::milliseconds(16));
-
-    // Progress indicator (every 30 frames for shorter test)
-    if (frame % 30 == 0) {
-      printf(
-          "  Frame %d: music_time=%.2fs, tempo=%.2fx, consumed=%d frames, "
-          "underruns=%d\r",
-          frame, music_time, tempo_scale,
-          jittered_backend.get_total_frames_consumed(),
-          jittered_backend.get_underrun_count());
-      fflush(stdout);
-    }
   }
   printf("\n");