docs(timeline-editor): Reference SEQUENCE.md and add priority editing task

Documentation improvements:
1. Added reference to doc/SEQUENCE.md in README
2. Added comment in parser pointing to format spec
3. Updated README with complete format examples including:
   - Priority modifiers (+, =, -)
   - Time notation (beats vs seconds)
   - Optional sequence names
   - BPM declaration

New feature task (Phase 1):
Task 1.2b: Priority Editing UI (HIGH PRIORITY, 6-8h)
- Edit sequence priority (0-9 for scene, 10+ for post-processing)
- Toggle effect priority modifiers (+/=/-)
- Visual priority indicators and z-order visualization
- Computed priority display
- Priority conflict warnings

Implementation details:
- Radio buttons for priority modifiers
- Up/down buttons for sequence priority
- Color-coded priority levels
- Priority badges on timeline items
- Automatic priority recalculation

Rationale:
Priority control is essential for proper render order and z-stacking.
Currently, priorities are shown but not easily editable. This task
makes priority a first-class editable property.

Updated effort estimates:
- Phase 1: 28-36 hours (was 22-28)
- Total: ~117-161 hours for full feature set

0 files changed, 0 insertions, 0 deletions