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, 15 insertions, 0 deletions

diff --git a/src/generated/test_demo_timeline.cc b/src/generated/test_demo_timeline.cc
new file mode 100644
index 0000000..75c5fe6
--- /dev/null
+++ b/src/generated/test_demo_timeline.cc
@@ -0,0 +1,15 @@
+// Auto-generated by seq_compiler. Do not edit.
+#include "gpu/demo_effects.h"
+#include "gpu/effect.h"
+
+float GetDemoDuration() {
+  return 16.000000f;
+}
+
+void LoadTimeline(MainSequence& main_seq, WGPUDevice device, WGPUQueue queue, WGPUTextureFormat format) {
+  {
+    auto seq = std::make_shared<Sequence>();
+    seq->add_effect(std::make_shared<FlashEffect>(device, queue, format), 0.0f, 16.0f, 0);
+    main_seq.add_sequence(seq, 0.0f, 0);
+  }
+}