feat(audio): Complete Phase 3 - Migrate main.cc to AudioEngine (Task #56)

Migrated production code (main.cc) to use AudioEngine instead of directly
calling synth_init() and tracker_init(), eliminating initialization order
dependencies in the demo entry point.

Changes:
- Added #include "audio/audio_engine.h" to main.cc
- Replaced synth_init() + tracker_init() with AudioEngine::init()
- Replaced tracker_update(g_music_time) with g_audio_engine.update(g_music_time)
- Preserved direct synth calls (synth_register_spectrogram, synth_get_output_peak)
  as these are valid API usage

Results:
- All 20 tests pass (100% pass rate)
- Demo runs successfully without crashes
- Initialization order fragility eliminated in production code
- Test suite time: 8.13s (unchanged)

Known Technical Debt (deferred to Phase 4):
- audio_init() still calls synth_init() internally for backwards compatibility
- This causes double initialization (harmless but fragile)
- Some tests rely on this behavior
- Will be cleaned up in Phase 4 with other compatibility shims

handoff(Claude): Completed Task #56 Phase 3 - production code now uses AudioEngine.
Phase 4 (Cleanup) remains: remove old global functions, update remaining tests,
remove backwards compatibility shims, update documentation.

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

1 files changed, 8 insertions, 4 deletions

diff --git a/src/main.cc b/src/main.cc
index 42adf6b..4542824 100644
--- a/src/main.cc
+++ b/src/main.cc
@@ -4,6 +4,7 @@
 
 #include "3d/renderer.h"
 #include "audio/audio.h"
+#include "audio/audio_engine.h"
 #include "audio/gen.h"
 #include "audio/synth.h"
 #include "audio/tracker.h"
@@ -101,9 +102,12 @@ int main(int argc, char** argv) {
   }
 #endif
 
+  // Initialize audio backend (miniaudio device)
   audio_init();
-  synth_init();
-  tracker_init();
+
+  // Initialize audio engine (synth + tracker unified management)
+  static AudioEngine g_audio_engine;
+  g_audio_engine.init();
 
   // Still keep the dynamic tone for bass (can be integrated into tracker too)
   const float* g_spec_buffer_a = generate_tone(nullptr, 110.0f); // A2
@@ -182,7 +186,7 @@ int main(int argc, char** argv) {
     }
 
     // Pass music_time (not physical time) to tracker
-    tracker_update(g_music_time);
+    g_audio_engine.update(g_music_time);
 
     // Fill ring buffer with upcoming audio (look-ahead rendering)
     // CRITICAL: Scale dt by tempo to render enough audio during acceleration/deceleration
@@ -209,7 +213,7 @@ int main(int argc, char** argv) {
 
   // PRE-FILL: Fill ring buffer with initial 200ms before starting audio device
   // This prevents underrun on first callback
-  tracker_update(g_music_time);
+  g_audio_engine.update(g_music_time);
   audio_render_ahead(g_music_time, 1.0f / 60.0f);  // Fill buffer with lookahead
 
   // Start audio (or render to WAV file)