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, 223 insertions, 0 deletions

diff --git a/test_demo_README.md b/test_demo_README.md
new file mode 100644
index 0000000..f84f972
--- /dev/null
+++ b/test_demo_README.md
@@ -0,0 +1,223 @@
+# test_demo - Audio/Visual Synchronization Debug Tool
+
+## Overview
+
+A minimal standalone tool for debugging audio/visual synchronization in the demo without the complexity of the full demo64k timeline.
+
+## Features
+
+- **Simple Drum Beat**: Kick-snare pattern with crash every 4th bar
+- **NOTE_A4 Reference Tone**: 440 Hz tone plays at start of each bar for testing
+- **Screen Flash Effect**: Visual feedback synchronized to audio peaks
+- **16 Second Duration**: 8 bars at 120 BPM
+- **Variable Tempo Mode**: Tests tempo scaling (1.0x ↔ 1.5x and 1.0x ↔ 0.66x)
+- **Peak Logging**: Export audio peaks to file for gnuplot visualization
+
+## Building
+
+```bash
+cmake --build build --target test_demo
+```
+
+## Usage
+
+### Basic Usage
+
+```bash
+build/test_demo
+```
+
+### Command-Line Options
+
+```
+  --help              Show help message and exit
+  --fullscreen        Run in fullscreen mode
+  --resolution WxH    Set window resolution (e.g., 1024x768)
+  --tempo             Enable tempo variation test mode
+  --log-peaks FILE    Log audio peaks to FILE for gnuplot visualization
+```
+
+### Examples
+
+#### Run in fullscreen
+```bash
+build/test_demo --fullscreen
+```
+
+#### Custom resolution with tempo testing
+```bash
+build/test_demo --resolution 1024x768 --tempo
+```
+
+#### Log audio peaks for analysis
+```bash
+build/test_demo --log-peaks peaks.txt
+```
+
+After running, visualize with gnuplot:
+```bash
+gnuplot -p -e "set xlabel 'Time (s)'; set ylabel 'Peak'; plot 'peaks.txt' using 1:3 with lines title 'Raw Peak'"
+```
+
+## Keyboard Controls
+
+- **ESC**: Exit the demo
+- **F**: Toggle fullscreen
+
+## Audio Pattern
+
+The demo plays a repeating drum pattern:
+
+```
+Bar 1-2: Kick-Snare-Kick-Snare (with A4 note)
+Bar 3:   Kick+Crash-Snare-Kick-Snare (with A4 note)  ← Crash landmark
+Bar 4-6: Kick-Snare-Kick-Snare (with A4 note)
+Bar 7:   Kick+Crash-Snare-Kick-Snare (with A4 note)  ← Crash landmark
+Bar 8:   Kick-Snare-Kick-Snare (with A4 note)
+```
+
+**Timing:**
+- 120 BPM (2 beats per second)
+- 1 bar = 4 beats = 2 seconds
+- Total duration: 8 bars = 16 seconds
+
+**Crash Landmarks:**
+- First crash at T=4.0s (bar 3, beat 8)
+- Second crash at T=12.0s (bar 7, beat 24)
+
+## Tempo Test Mode (`--tempo`)
+
+When enabled with `--tempo` flag, the demo alternates tempo scaling every bar:
+
+- **Even bars (0, 2, 4, 6)**: Accelerate from 1.0x → 1.5x
+- **Odd bars (1, 3, 5, 7)**: Decelerate from 1.0x → 0.66x
+
+This tests the variable tempo system where music time advances independently of physical time:
+- `music_time += dt * tempo_scale`
+- Pattern triggering respects tempo scaling
+- Audio samples play at normal pitch (no pitch shifting)
+
+**Console Output with --tempo:**
+```
+[T=0.50, MusicT=0.56, Beat=1, Frac=0.12, Peak=0.72, Tempo=1.25x]
+```
+
+**Expected Behavior:**
+- Physical time always advances at 1:1 (16 seconds = 16 seconds)
+- Music time advances faster during acceleration, slower during deceleration
+- By end of demo: Music time > Physical time (due to net acceleration)
+
+## Peak Logging Format
+
+The `--log-peaks` option writes a text file with three columns:
+
+```
+# Audio peak log from test_demo
+# To plot with gnuplot:
+#   gnuplot -p -e "set xlabel 'Time (s)'; set ylabel 'Peak'; plot 'peaks.txt' using 1:3 with lines title 'Raw Peak'"
+# Columns: beat_number clock_time raw_peak
+#
+0 0.000000 0.850000
+1 0.500000 0.720000
+2 1.000000 0.800000
+...
+```
+
+**Columns:**
+1. **beat_number**: Beat index (0, 1, 2, ...)
+2. **clock_time**: Physical time in seconds
+3. **raw_peak**: Audio peak value (0.0-1.0+)
+
+**Use Cases:**
+- Verify audio/visual synchronization timing
+- Detect clipping (peak > 1.0)
+- Analyze tempo scaling effects
+- Compare expected vs actual beat times
+
+## Files
+
+- **`src/test_demo.cc`**: Main executable (~220 lines)
+- **`assets/test_demo.track`**: Drum pattern and NOTE_A4 definition
+- **`assets/test_demo.seq`**: Visual timeline (FlashEffect)
+- **`src/generated/test_demo_timeline.cc`**: Generated timeline (auto)
+- **`src/generated/test_demo_music.cc`**: Generated music data (auto)
+
+## Verification Checklist
+
+### Normal Mode
+- [ ] Visual flashes occur every ~0.5 seconds
+- [ ] Audio plays (kick-snare drum pattern audible)
+- [ ] A4 tone (440 Hz) audible at start of each bar
+- [ ] Synchronization: Flash happens simultaneously with kick hit
+- [ ] Crash landmark: Larger flash + cymbal crash at T=4.0s and T=12.0s
+- [ ] Auto-exit: Demo stops cleanly at 16 seconds
+- [ ] Console timing: Peak values spike when kicks hit (Peak > 0.7)
+- [ ] No audio glitches: Smooth playback, no stuttering
+
+### Tempo Test Mode
+- [ ] Bar 0 accelerates (1.0x → 1.5x)
+- [ ] Bar 1 decelerates (1.0x → 0.66x)
+- [ ] Music time drifts from physical time
+- [ ] Audio still syncs with flashes
+- [ ] Smooth transitions at bar boundaries
+- [ ] Physical time = 16s at end
+- [ ] Music time > 16s at end
+- [ ] Console shows tempo value
+
+### Peak Logging
+- [ ] File created when `--log-peaks` specified
+- [ ] Contains beat_number, clock_time, raw_peak columns
+- [ ] Gnuplot command in header comment
+- [ ] One row per beat (32 rows for 16 seconds)
+- [ ] Peak values match console output
+- [ ] Gnuplot visualization works
+
+## Troubleshooting
+
+**Flash appears before audio:**
+- Cause: Audio latency too high
+- Fix: Reduce ring buffer size in `ring_buffer.h`
+
+**Flash appears after audio:**
+- Cause: Ring buffer under-filled
+- Fix: Increase pre-fill duration in `audio_render_ahead()`
+
+**No flash at all:**
+- Cause: Peak threshold not reached
+- Check: Console shows Peak > 0.7
+- Fix: Increase `visual_peak` multiplier in code (currently 8.0×)
+
+**A4 note not audible:**
+- Cause: NOTE_A4 volume too low or procedural generation issue
+- Check: Console shows correct sample count (4 samples)
+- Fix: Increase volume in test_demo.track (currently 0.5)
+
+**Peak log file empty:**
+- Cause: Demo exited before first beat
+- Check: File has header comments
+- Fix: Ensure demo runs for at least 0.5 seconds
+
+## Design Rationale
+
+**Why separate from demo64k?**
+- Isolated testing environment
+- No timeline complexity
+- Faster iteration cycles
+- Independent verification
+
+**Why use main demo assets?**
+- Avoid asset system conflicts (AssetId enum collision)
+- Reuse existing samples
+- No size overhead
+- Simpler CMake integration
+
+**Why 16 seconds?**
+- Long enough for verification (8 bars)
+- Short enough for quick tests
+- Crash landmarks at 25% and 75% for easy reference
+
+**Why NOTE_A4?**
+- Standard reference tone (440 Hz)
+- Easily identifiable pitch
+- Tests procedural note generation
+- Minimal code impact