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, 18 insertions, 10 deletions

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 350abdf..03ce641 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -77,8 +77,8 @@ elseif (NOT DEMO_CROSS_COMPILE_WIN32)
     list(APPEND DEMO_LIBS pthread m dl)
 endif()
 
-#-- - Source Groups -- - 
-set(AUDIO_SOURCES src/audio/audio.cc src/audio/gen.cc src/audio/fdct.cc src/audio/idct.cc src/audio/window.cc src/audio/synth.cc src/audio/tracker.cc)
+#-- - Source Groups -- -
+set(AUDIO_SOURCES src/audio/audio.cc src/audio/miniaudio_backend.cc src/audio/gen.cc src/audio/fdct.cc src/audio/idct.cc src/audio/window.cc src/audio/synth.cc src/audio/tracker.cc)
 set(PROCEDURAL_SOURCES src/procedural/generator.cc)
 set(GPU_SOURCES
     src/gpu/gpu.cc
@@ -264,19 +264,27 @@ endif()
 #-- - Tests -- - 
 enable_testing()
 if(DEMO_BUILD_TESTS)
-    add_demo_test(test_window HammingWindowTest src/tests/test_window.cc)
-    target_link_libraries(test_window PRIVATE audio ${DEMO_LIBS})
+    add_demo_test(test_window HammingWindowTest src/tests/test_window.cc ${GEN_DEMO_CC})
+    target_link_libraries(test_window PRIVATE audio util procedural ${DEMO_LIBS})
+    add_dependencies(test_window generate_demo_assets)
 
     add_demo_test(test_maths MathUtilsTest src/tests/test_maths.cc)
 
-    add_demo_test(test_synth SynthEngineTest src/tests/test_synth.cc)
-    target_link_libraries(test_synth PRIVATE audio ${DEMO_LIBS})
+    add_demo_test(test_synth SynthEngineTest src/tests/test_synth.cc ${GEN_DEMO_CC})
+    target_link_libraries(test_synth PRIVATE audio util procedural ${DEMO_LIBS})
+    add_dependencies(test_synth generate_demo_assets)
 
-    add_demo_test(test_dct DctTest src/tests/test_dct.cc)
-    target_link_libraries(test_dct PRIVATE audio ${DEMO_LIBS})
+    add_demo_test(test_dct DctTest src/tests/test_dct.cc ${GEN_DEMO_CC})
+    target_link_libraries(test_dct PRIVATE audio util procedural ${DEMO_LIBS})
+    add_dependencies(test_dct generate_demo_assets)
 
-    add_demo_test(test_audio_gen AudioGenTest src/tests/test_audio_gen.cc)
-    target_link_libraries(test_audio_gen PRIVATE audio ${DEMO_LIBS})
+    add_demo_test(test_audio_gen AudioGenTest src/tests/test_audio_gen.cc ${GEN_DEMO_CC})
+    target_link_libraries(test_audio_gen PRIVATE audio util procedural ${DEMO_LIBS})
+    add_dependencies(test_audio_gen generate_demo_assets)
+
+    add_demo_test(test_audio_backend AudioBackendTest src/tests/test_audio_backend.cc ${GEN_DEMO_CC})
+    target_link_libraries(test_audio_backend PRIVATE audio util procedural ${DEMO_LIBS})
+    add_dependencies(test_audio_backend generate_demo_assets)
 
     add_demo_test(test_tracker TrackerSystemTest src/tests/test_tracker.cc ${GEN_DEMO_CC} ${GENERATED_MUSIC_DATA_CC})
     target_link_libraries(test_tracker PRIVATE audio util procedural ${DEMO_LIBS})