feat(audio): Implement mock audio backend for testing (Task #51.2)

Created MockAudioBackend for deterministic audio event recording and
timing verification. Enables robust tracker synchronization testing.

Changes:
- Created VoiceTriggerEvent structure (timestamp, spec_id, volume, pan)
- Implemented MockAudioBackend with event recording capabilities
- Added time tracking: manual (advance_time) and automatic (on_frames_rendered)
- Created test_mock_backend.cc with 6 comprehensive test scenarios
- Verified synth integration and audio_render_silent compatibility
- Added to CMake test builds

All test infrastructure guarded by #if !defined(STRIP_ALL). Zero size
impact on production build. All 14 tests pass.

handoff(Claude): Task #51.2 complete, mock backend ready for tracker tests

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

1 files changed, 10 insertions, 1 deletions

diff --git a/TODO.md b/TODO.md
index a981f8e..71ed4ff 100644
--- a/TODO.md
+++ b/TODO.md
@@ -3,6 +3,15 @@
 This file tracks prioritized tasks with detailed attack plans.
 
 ## Recently Completed (February 4, 2026)
+- [x] **Task #51.2: Mock Audio Backend**:
+    - [x] **Event Recording**: Created `VoiceTriggerEvent` structure to capture timestamp, spectrogram_id, volume, and pan.
+    - [x] **MockAudioBackend Class**: Implemented test-only backend with event recording and time tracking capabilities.
+    - [x] **Time Management**: Added `advance_time()`, `set_time()`, and `get_current_time()` for deterministic testing.
+    - [x] **Frame Rendering Hook**: Implemented `on_frames_rendered()` to automatically update time based on audio frames (32kHz).
+    - [x] **Synth Integration**: Verified mock backend correctly captures voice triggers from synth engine.
+    - [x] **Comprehensive Tests**: Created `test_mock_backend.cc` with 6 test scenarios covering all mock functionality.
+    - [x] **Build Integration**: Added mock backend to test builds, all 14 tests pass.
+
 - [x] **Task #51.1: Audio Backend Abstraction**:
     - [x] **Interface Created**: Defined `AudioBackend` interface in `src/audio/audio_backend.h` with hooks for voice triggering and frame rendering.
     - [x] **Production Backend**: Moved miniaudio implementation to `MiniaudioBackend` class, maintaining backward compatibility.
@@ -73,7 +82,7 @@ This file tracks prioritized tasks with detailed attack plans.
 
 - [ ] **Task #51: Tracker Timing Verification**
     - [x] **Task #51.1: Audio Backend Abstraction**: Create an interface to separate audio output from synth logic, enabling testable backends.
-    - [ ] **Task #51.2: Mock Audio Backend**: Implement a test backend that records voice trigger events with precise timestamps.
+    - [x] **Task #51.2: Mock Audio Backend**: Implement a test backend that records voice trigger events with precise timestamps.
     - [ ] **Task #51.3: Tracker Test Suite**: Create `test_tracker.cc` to verify pattern triggering, timing accuracy, and synchronization.
     - [ ] **Task #51.4: Integration with Build**: Wire up tests to CMake and ensure they run in CI.