update doc, optimize spectral_editor

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 This file tracks prioritized tasks with detailed attack plans.
 
+## Recently Completed (February 7, 2026)
+
+- [x] **Audio Peak Measurement & Test Coverage Improvements** (February 7, 2026)
+    - [x] **Real-Time Peak Fix**: Fixed critical audio-visual sync bug where visual effects triggered ~400ms before audio was heard
+      - Root cause: Peak measured at ring buffer write time (synth_render) instead of playback time (audio callback)
+      - Solution: Added `get_realtime_peak()` to AudioBackend interface, implemented in MiniaudioBackend audio_callback
+      - Used exponential averaging: instant attack, 0.7 decay rate (1-second fade time)
+    - [x] **Peak Decay Optimization**: Fixed "test_demo just flashing" issue
+      - Old: 0.95 decay = 5.76 second fade (screen stayed white)
+      - New: 0.7 decay = 1.15 second fade (proper flash with smooth decay)
+    - [x] **SilentBackend Creation**: Test-only backend for audio.cc testing without hardware
+      - Created src/audio/backend/silent_backend.{h,cc}
+      - 7 comprehensive tests: lifecycle, peak control, tracking, playback time, buffer management
+      - Significantly improved audio.cc test coverage
+    - [x] **Backend Reorganization**: Moved all backends to src/audio/backend/ subdirectory
+      - Cleaner code organization, updated all includes and CMake paths
+    - [x] **Dead Code Removal**: Removed unused `register_spec_asset()` function
+    - **Result**: All 28 tests passing, perfect audio-visual sync, improved test coverage
+
 ## Recently Completed (February 6, 2026)
 
 - [x] **Critical Shader Bug Fixes & Test Infrastructure** (February 6, 2026)