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, 31 insertions, 0 deletions

diff --git a/src/audio/miniaudio_backend.h b/src/audio/miniaudio_backend.h
new file mode 100644
index 0000000..d46a0c5
--- /dev/null
+++ b/src/audio/miniaudio_backend.h
@@ -0,0 +1,31 @@
+// This file is part of the 64k demo project.
+// It implements the production audio backend using miniaudio.
+// This is the default backend for the final build.
+
+#pragma once
+
+#include "audio_backend.h"
+#include "miniaudio.h"
+
+// Production audio backend using miniaudio library
+// Manages real hardware audio device and playback
+class MiniaudioBackend : public AudioBackend {
+ public:
+  MiniaudioBackend();
+  ~MiniaudioBackend() override;
+
+  void init() override;
+  void start() override;
+  void shutdown() override;
+
+  // Get the underlying miniaudio device (for internal use)
+  ma_device* get_device() { return &device_; }
+
+ private:
+  ma_device device_;
+  bool initialized_;
+
+  // Static callback required by miniaudio C API
+  static void audio_callback(ma_device* pDevice, void* pOutput,
+                             const void* pInput, ma_uint32 frameCount);
+};