feat(audio): Implement audio backend abstraction (Task #51.1)

Created interface-based audio backend system to enable testing without
hardware. This is the foundation for robust tracker timing verification.

Changes:
- Created AudioBackend interface with init/start/shutdown methods
- Added test-only hooks: on_voice_triggered() and on_frames_rendered()
- Moved miniaudio implementation to MiniaudioBackend class
- Refactored audio.cc to use backend abstraction with auto-fallback
- Added time tracking to synth.cc (elapsed time from rendered frames)
- Created test_audio_backend.cc to verify backend injection works
- Fixed audio test linking to include util/procedural dependencies

All test infrastructure guarded by #if !defined(STRIP_ALL) for zero
size impact on final build. Production path unchanged, 100% backward
compatible. All 13 tests pass.

handoff(Claude): Task #51.1 complete, audio backend abstraction ready

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

1 files changed, 44 insertions, 25 deletions

diff --git a/src/audio/audio.cc b/src/audio/audio.cc
index d08a2fa..6ee9782 100644
--- a/src/audio/audio.cc
+++ b/src/audio/audio.cc
@@ -1,16 +1,35 @@
 // This file is part of the 64k demo project.
 // It manages the low-level audio device and high-level audio state.
-// Implementation uses miniaudio for cross-platform support.
+// Now uses backend abstraction for testability.
 
 #include "audio.h"
+#include "audio_backend.h"
+#include "miniaudio_backend.h"
+#include "synth.h"
 #include "util/asset_manager.h"
 
 #define MINIAUDIO_IMPLEMENTATION
 #include "miniaudio.h"
-#include "synth.h"
 
 #include <stdio.h>
 
+// Global backend pointer for audio abstraction
+static AudioBackend* g_audio_backend = nullptr;
+static MiniaudioBackend g_default_backend;
+static bool g_using_default_backend = false;
+
+#if !defined(STRIP_ALL)
+// Allow tests to inject a custom backend
+void audio_set_backend(AudioBackend* backend) {
+  g_audio_backend = backend;
+}
+
+// Get current backend (for tests)
+AudioBackend* audio_get_backend() {
+  return g_audio_backend;
+}
+#endif /* !defined(STRIP_ALL) */
+
 int register_spec_asset(AssetId id) {
   size_t size;
   const uint8_t* data = GetAsset(id, &size);
@@ -28,36 +47,24 @@ int register_spec_asset(AssetId id) {
   return synth_register_spectrogram(&spec);
 }
 
-static ma_device g_device;
-
-void audio_data_callback(ma_device* pDevice, void* pOutput, const void* pInput,
-                         ma_uint32 frameCount) {
-  (void)pInput;
-  float* fOutput = (float*)pOutput;
-  synth_render(fOutput, (int)frameCount);
-}
-
 void audio_init() {
   synth_init();
 
-  ma_device_config config = ma_device_config_init(ma_device_type_playback);
-  config.playback.format = ma_format_f32;
-  config.playback.channels = 2;
-  config.sampleRate = 32000;
-  config.dataCallback = audio_data_callback;
-
-  if (ma_device_init(NULL, &config, &g_device) != MA_SUCCESS) {
-    printf("Failed to open playback device.\n");
-    return;
+  // Use default backend if none set
+  if (g_audio_backend == nullptr) {
+    g_audio_backend = &g_default_backend;
+    g_using_default_backend = true;
   }
+
+  g_audio_backend->init();
 }
 
 void audio_start() {
-  if (ma_device_start(&g_device) != MA_SUCCESS) {
-    printf("Failed to start playback device.\n");
-    ma_device_uninit(&g_device);
+  if (g_audio_backend == nullptr) {
+    printf("Cannot start: audio not initialized.\n");
     return;
   }
+  g_audio_backend->start();
 }
 
 #if !defined(STRIP_ALL)
@@ -72,6 +79,11 @@ void audio_render_silent(float duration_sec) {
         (total_frames > chunk_size) ? chunk_size : total_frames;
     synth_render(buffer, frames_to_render);
     total_frames -= frames_to_render;
+
+    // Notify backend of frames rendered (for mock tracking)
+    if (g_audio_backend != nullptr) {
+      g_audio_backend->on_frames_rendered(frames_to_render);
+    }
   }
 }
 #endif /* !defined(STRIP_ALL) */
@@ -80,7 +92,14 @@ void audio_update() {
 }
 
 void audio_shutdown() {
-  ma_device_stop(&g_device);
-  ma_device_uninit(&g_device);
+  if (g_audio_backend != nullptr) {
+    g_audio_backend->shutdown();
+  }
   synth_shutdown();
+
+  // Clear backend pointer if using default
+  if (g_using_default_backend) {
+    g_audio_backend = nullptr;
+    g_using_default_backend = false;
+  }
 }