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>

2 files changed, 92 insertions, 0 deletions

diff --git a/src/generated/test_demo_music.cc b/src/generated/test_demo_music.cc
new file mode 100644
index 0000000..3fdd2a1
--- /dev/null
+++ b/src/generated/test_demo_music.cc
@@ -0,0 +1,77 @@
+// Generated by tracker_compiler. Do not edit.
+
+#include "audio/tracker.h"
+
+#include "generated/assets.h"
+
+const NoteParams g_tracker_samples[] = {
+    { 0 }, // ASSET_KICK_1 (ASSET)
+    { 0 }, // ASSET_SNARE_1 (ASSET)
+    { 0 }, // ASSET_CRASH_1 (ASSET)
+    { 440.0f, 0.50f, 1.0f, 0.01f, 0.0f, 0.0f, 0.0f, 3, 0.6f, 0.0f, 0.0f }, // NOTE_A4
+};
+const uint32_t g_tracker_samples_count = 4;
+
+const AssetId g_tracker_sample_assets[] = {
+  AssetId::ASSET_KICK_1,
+  AssetId::ASSET_SNARE_1,
+  AssetId::ASSET_CRASH_1,
+  AssetId::ASSET_LAST_ID,
+};
+
+static const TrackerEvent PATTERN_EVENTS_drums_basic[] = {
+    { 0.0f, 0, 1.0f, 0.0f },
+    { 0.0f, 3, 0.5f, 0.0f },
+    { 1.0f, 1, 0.9f, 0.0f },
+    { 2.0f, 0, 1.0f, 0.0f },
+    { 3.0f, 1, 0.9f, 0.0f },
+};
+static const TrackerEvent PATTERN_EVENTS_drums_with_crash[] = {
+    { 0.0f, 0, 1.0f, 0.0f },
+    { 0.0f, 2, 0.9f, 0.0f },
+    { 0.0f, 3, 0.5f, 0.0f },
+    { 1.0f, 1, 0.9f, 0.0f },
+    { 2.0f, 0, 1.0f, 0.0f },
+    { 3.0f, 1, 0.9f, 0.0f },
+};
+
+const TrackerPattern g_tracker_patterns[] = {
+    { PATTERN_EVENTS_drums_basic, 5, 4.0f }, // drums_basic
+    { PATTERN_EVENTS_drums_with_crash, 6, 4.0f }, // drums_with_crash
+};
+const uint32_t g_tracker_patterns_count = 2;
+
+static const TrackerPatternTrigger SCORE_TRIGGERS[] = {
+    { 0.0f, 0 },
+    { 2.0f, 0 },
+    { 4.0f, 1 },
+    { 6.0f, 0 },
+    { 8.0f, 0 },
+    { 10.0f, 0 },
+    { 12.0f, 1 },
+    { 14.0f, 0 },
+};
+
+const TrackerScore g_tracker_score = {
+    SCORE_TRIGGERS, 8, 120.0f
+};
+
+// ============================================================
+// RESOURCE USAGE ANALYSIS (for synth.h configuration)
+// ============================================================
+// Total samples: 4 (3 assets + 1 generated notes)
+// Max simultaneous pattern triggers: 1
+// Estimated max polyphony: 5 voices
+// 
+// REQUIRED (minimum to avoid pool exhaustion):
+//   MAX_VOICES: 5
+//   MAX_SPECTROGRAMS: 8 (no caching)
+// 
+// RECOMMENDED (with 50% safety margin):
+//   MAX_VOICES: 10
+//   MAX_SPECTROGRAMS: 12 (no caching)
+// 
+// NOTE: With spectrogram caching by note parameters,
+//       MAX_SPECTROGRAMS could be reduced to ~4
+// ============================================================
+
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);
+  }
+}