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>

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+# Task Completion Summary
+
+All 5 tasks have been successfully completed:
+
+## ✅ Task #1: Fix real-time audio peak measurement for visual sync
+**Issue:** Visual effects triggered 400ms before audio was heard due to peak measurement at ring buffer write time.
+
+**Solution:**
+- Added `get_realtime_peak()` to AudioBackend interface
+- Implemented real-time peak measurement in audio callback
+- Updated main.cc and test_demo.cc to use `audio_get_realtime_peak()`
+- Fixed peak decay rate from 0.95 (6 second fade) to 0.7 (1 second fade)
+
+**Files Modified:**
+- src/audio/audio_backend.h
+- src/audio/backend/miniaudio_backend.{h,cc}
+- src/audio/backend/{mock,wav_dump,jittered,silent}_backend.{h,cc}
+- src/audio/audio.{h,cc}
+- src/main.cc
+- src/test_demo.cc
+- src/tests/test_audio_backend.cc
+
+**Result:** Audio-visual synchronization now accurate, visual effects fade smoothly.
+
+---
+
+## ✅ Task #2: Create SilentBackend for audio testing
+**Goal:** Test audio.cc functionality without hardware.
+
+**Implementation:**
+- Created SilentBackend class implementing AudioBackend interface
+- No audio output (silent), pure inspection/testing
+- Controllable peak for testing edge cases
+- Tracks frames rendered and voice triggers
+
+**Files Created:**
+- src/audio/backend/silent_backend.{h,cc}
+- src/tests/test_silent_backend.cc
+
+**Tests Added:** 7 comprehensive tests covering:
+- Lifecycle (init/start/shutdown)
+- Peak control
+- Frame/voice tracking
+- Playback time
+- Buffer management
+- audio_update()
+
+**Result:** 28/28 tests pass, improved audio.cc test coverage.
+
+---
+
+## ✅ Task #3: Reorganize audio backends to audio/backend/ directory
+**Goal:** Improve codebase organization.
+
+**Changes:**
+- Created src/audio/backend/ directory
+- Moved all backend implementations:
+  - miniaudio_backend.{h,cc}
+  - mock_audio_backend.{h,cc}
+  - wav_dump_backend.{h,cc}
+  - jittered_audio_backend.{h,cc}
+  - silent_backend.{h,cc}
+- Kept audio_backend.h in src/audio/ (interface)
+- Updated all #include paths
+- Updated CMakeLists.txt paths
+
+**Files Modified:**
+- CMakeLists.txt (AUDIO_SOURCES and test targets)
+- src/audio/audio.cc
+- src/main.cc
+- src/tests/*.cc (all backend-using tests)
+- All backend .{h,cc} files (relative includes)
+
+**Result:** Cleaner directory structure, all 28 tests pass.
+
+---
+
+## ✅ Task #4: Remove dead code (register_spec_asset)
+**Goal:** Remove unused function.
+
+**Analysis:** Function was never called anywhere in codebase.
+
+**Changes:**
+- Removed declaration from src/audio/audio.h
+- Removed implementation from src/audio/audio.cc
+
+**Result:** Code cleanup complete, all 28 tests pass.
+
+---
+
+## ✅ Task #5: Add comprehensive tests for audio.cc coverage
+**Status:** Completed via Task #2 (SilentBackend tests).
+
+**Coverage Improvements:**
+- audio_init/start/shutdown: ✓ tested
+- audio_get_realtime_peak: ✓ tested
+- audio_render_ahead: ✓ tested
+- audio_update: ✓ tested
+- audio_get_playback_time: ✓ tested
+- Buffer management: ✓ tested
+
+**Result:** Significant audio.cc coverage improvement.
+
+---
+
+## Final Status
+
+**Tests:** 28/28 passing (100%)
+**Build:** Clean, no warnings
+**Executables:** demo64k and test_demo build successfully
+**Code Quality:** Improved organization, dead code removed, comprehensive tests
+
+**Notable Improvements:**
+1. Audio-visual sync fixed (400ms timing issue resolved)
+2. Peak decay optimized (6s → 1s fade time)
+3. Test infrastructure enhanced (SilentBackend)
+4. Code organization improved (backend/ subdirectory)
+5. Test coverage significantly improved
+
+**Ready for:** Further development or testing