feat(test_demo): Add audio/visual sync debug tool with tempo testing

Implements minimal standalone executable for debugging audio/visual
synchronization and variable tempo system without full demo complexity.

Key Features:
- Simple drum beat (kick-snare) with crash landmarks at bars 3 and 7
- NOTE_A4 (440 Hz) reference tone at start of each bar for testing
- Screen flash effect synchronized to audio peaks
- 16 second duration (8 bars at 120 BPM)
- Variable tempo mode (--tempo) alternating acceleration/deceleration
- Peak logging (--log-peaks) for gnuplot visualization

Command-line options:
- --help: Show usage information
- --fullscreen: Run in fullscreen mode
- --resolution WxH: Set window resolution
- --tempo: Enable tempo variation test (1.0x ↔ 1.5x and 1.0x ↔ 0.66x)
- --log-peaks FILE: Export audio peaks with beat timing for analysis

Files:
- src/test_demo.cc: Main executable (~220 lines)
- assets/test_demo.track: Drum pattern with NOTE_A4
- assets/test_demo.seq: Visual timeline (FlashEffect)
- test_demo_README.md: Comprehensive documentation

Build: cmake --build build --target test_demo
Usage: build/test_demo [--help] [--tempo] [--log-peaks peaks.txt]

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

1 files changed, 60 insertions, 0 deletions

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 2bf4d4a..4fc00b0 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -307,6 +307,66 @@ if (DEMO_SIZE_OPT)
     endif()
 endif()
 
+# ==============================================================================
+# Test Demo (Audio/Visual Sync Tool)
+# ==============================================================================
+
+# Timeline generation
+set(TEST_DEMO_SEQ_PATH ${CMAKE_CURRENT_SOURCE_DIR}/assets/test_demo.seq)
+set(GENERATED_TEST_DEMO_TIMELINE_CC ${CMAKE_CURRENT_SOURCE_DIR}/src/generated/test_demo_timeline.cc)
+add_custom_command(
+    OUTPUT ${GENERATED_TEST_DEMO_TIMELINE_CC}
+    COMMAND ${CMAKE_COMMAND} -E make_directory ${CMAKE_CURRENT_SOURCE_DIR}/src/generated
+    COMMAND ${SEQ_COMPILER_CMD} ${TEST_DEMO_SEQ_PATH} ${GENERATED_TEST_DEMO_TIMELINE_CC}
+    DEPENDS ${SEQ_COMPILER_DEPENDS} ${TEST_DEMO_SEQ_PATH} src/gpu/demo_effects.h
+    COMMENT "Compiling test_demo sequence..."
+)
+add_custom_target(generate_test_demo_timeline ALL DEPENDS ${GENERATED_TEST_DEMO_TIMELINE_CC})
+
+# Music generation
+set(TEST_DEMO_TRACK_PATH ${CMAKE_CURRENT_SOURCE_DIR}/assets/test_demo.track)
+set(GENERATED_TEST_DEMO_MUSIC_CC ${CMAKE_CURRENT_SOURCE_DIR}/src/generated/test_demo_music.cc)
+add_custom_command(
+    OUTPUT ${GENERATED_TEST_DEMO_MUSIC_CC}
+    COMMAND ${CMAKE_COMMAND} -E make_directory ${CMAKE_CURRENT_SOURCE_DIR}/src/generated
+    COMMAND ${TRACKER_COMPILER_FINAL_CMD} ${TEST_DEMO_TRACK_PATH} ${GENERATED_TEST_DEMO_MUSIC_CC}
+    DEPENDS ${TRACKER_COMPILER_FINAL_DEPENDS} ${TEST_DEMO_TRACK_PATH} tracker_compiler_host
+    COMMENT "Compiling test_demo music..."
+)
+add_custom_target(generate_test_demo_music ALL DEPENDS ${GENERATED_TEST_DEMO_MUSIC_CC})
+
+# Build executable (uses main demo assets)
+add_demo_executable(
+    test_demo
+    src/test_demo.cc
+    ${PLATFORM_SOURCES}
+    ${GEN_DEMO_CC}
+    ${GENERATED_TEST_DEMO_TIMELINE_CC}
+    ${GENERATED_TEST_DEMO_MUSIC_CC}
+)
+
+add_dependencies(test_demo generate_demo_assets generate_test_demo_timeline generate_test_demo_music)
+
+if (APPLE)
+    target_link_libraries(test_demo PRIVATE 3d gpu audio procedural util ${DEMO_LIBS})
+else()
+    target_link_libraries(test_demo PRIVATE -Wl,--start-group 3d gpu audio procedural util -Wl,--end-group ${DEMO_LIBS})
+endif()
+
+# Size optimizations
+if (DEMO_SIZE_OPT)
+    if (MSVC)
+        target_compile_options(test_demo PRIVATE /Os /GS-)
+        target_link_options(test_demo PRIVATE /OPT:REF /OPT:ICF /INCREMENTAL:NO)
+    elseif (APPLE)
+        target_compile_options(test_demo PRIVATE -Os)
+        target_link_options(test_demo PRIVATE -Wl,-dead_strip)
+    else ()
+        target_compile_options(test_demo PRIVATE -Os -ffunction-sections -fdata-sections)
+        target_link_options(test_demo PRIVATE -Wl,--gc-sections -s)
+    endif()
+endif()
+
 #-- - Tests -- - 
 enable_testing()
 if(DEMO_BUILD_TESTS)