feat(audio): Complete Phase 2 - Migrate tests to AudioEngine (Task #56)

Migrated all tracker-related tests to use AudioEngine instead of directly
calling synth_init() and tracker_init(), eliminating fragile initialization
order dependencies.

Tests Migrated:
- test_tracker.cc: Basic tracker functionality
- test_tracker_timing.cc: Timing verification with MockAudioBackend (7 tests)
- test_variable_tempo.cc: Variable tempo scaling (6 tests)
- test_wav_dump.cc: WAV dump backend verification

Migration Pattern:
- Added AudioEngine include to all test files
- Replaced synth_init() + tracker_init() with AudioEngine::init()
- Replaced tracker_update(time) with engine.update(time)
- Added engine.shutdown() at end of each test function
- Preserved audio_init()/audio_shutdown() where needed for backends

Results:
- All 20 tests pass (100% pass rate)
- Test suite time: 8.13s (slightly faster)
- No regressions in test behavior
- Cleaner API with single initialization entry point

Next Steps (Phase 3):
- Migrate main.cc and production code to use AudioEngine
- Add backwards compatibility shims during transition

handoff(Claude): Completed Task #56 Phase 2 - all tracker tests now use
AudioEngine. The initialization order fragility is eliminated in test code.
Ready for Phase 3 (production integration).

1 files changed, 7 insertions, 6 deletions

diff --git a/src/tests/test_wav_dump.cc b/src/tests/test_wav_dump.cc
index f350330..c68578b 100644
--- a/src/tests/test_wav_dump.cc
+++ b/src/tests/test_wav_dump.cc
@@ -2,8 +2,7 @@
 // Regression test for WAV dump backend to prevent format mismatches.
 
 #include "audio/audio.h"
-#include "audio/synth.h"
-#include "audio/tracker.h"
+#include "audio/audio_engine.h"
 #include "audio/wav_dump_backend.h"
 #include <assert.h>
 #include <stdio.h>
@@ -42,21 +41,23 @@ void test_wav_format_matches_live_audio() {
   // Initialize audio system (calls synth_init internally)
   audio_init();
 
-  // Initialize tracker AFTER audio_init to ensure spectrograms stay registered
-  tracker_init();
+  // Initialize AudioEngine (replaces direct synth_init/tracker_init)
+  AudioEngine engine;
+  engine.init();
 
   // Manually trigger some audio for testing
-  tracker_update(0.0f); // Trigger patterns at t=0
+  engine.update(0.0f); // Trigger patterns at t=0
 
   // Render short duration (1 second = 60 updates @ 60Hz)
   for (int i = 0; i < 60; ++i) {
     float t = i / 60.0f;
-    tracker_update(t);
+    engine.update(t);
 
     // Simulate audio render (WavDumpBackend will handle this in start())
   }
 
   audio_start(); // This triggers the actual WAV rendering
+  engine.shutdown();
   audio_shutdown();
 
   // Read and verify WAV header