docs: Document test_demo milestone and recent project state

Updates PROJECT_CONTEXT.md with recently completed work (February 7, 2026):

**test_demo - Audio/Visual Sync Debug Tool:**
- Standalone minimal executable for sync debugging
- Drum beat with NOTE_A4 reference tone (440 Hz)
- Variable tempo mode (--tempo) for music time testing
- Peak logging: beat-aligned and fine-grained (~960 samples)
- Command-line options: --help, --fullscreen, --resolution, --log-peaks
- Error handling for invalid options
- 220 lines of code, comprehensive documentation
- Use cases: millisecond-precision sync verification, timing jitter detection

**CMake Configuration Summary:**
- Formatted display of all build options (ON/OFF status)
- Shows build type and compiler information
- Improves developer experience and debugging

**Code Quality:**
- Fixed deprecated sprintf warning in asset_packer.cc
- Replaced with snprintf for buffer safety

This captures the current stable state of the project with the new
debug tooling infrastructure in place.

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

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