feat(tools): Implement Task #57 - Interactive Timeline Editor

Created a functional web-based timeline editor for demo.seq files.

Features implemented:
✅ Load/save demo.seq files via browser file API
✅ Visual Gantt-style timeline with time markers
✅ Drag & drop sequences and effects along timeline
✅ Properties panel for editing timing, priorities, arguments
✅ Zoom controls (10% - 200%)
✅ Add/delete sequences and effects
✅ Real-time statistics (sequence count, effect count, duration)
✅ Dark theme matching VSCode aesthetic
✅ Color-coded items (sequences blue, effects gray)
✅ Selection highlighting

Technical details:
- Pure HTML/CSS/JavaScript (no dependencies, no build step)
- ~600 lines in single self-contained HTML file
- Works offline, no backend needed
- Parser converts demo.seq text → JavaScript objects
- Serializer converts JavaScript objects → demo.seq text

Usage:
1. Open tools/timeline_editor/index.html in browser
2. Load assets/demo.seq
3. Drag items, edit properties
4. Save modified file
5. Copy to assets/demo.seq and rebuild

Limitations (acceptable for v1):
- No undo/redo yet
- No effect creation UI (must use properties panel)
- No overlap detection warnings
- No snap-to-grid
- No preview rendering (intentional - editor only)

This provides a quick way to experiment with demo timing without
recompiling, and better visualization of sequence/effect relationships.

Size: ~25KB (HTML + embedded JS/CSS)
Status: Task #57 basic implementation complete

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