- Add 1-bar lead-in margin at timeline start (bar 0, time 0s at margin edge)
- Playhead position includes lead-in offset
- Bar numbers now 0-indexed (0, 1, 2...) in ruler and track grid
- Fix bar number clipping: CLIP_BUFFER increased from 200px to
max(400px, barWidth+200px) so bars past viewport edge render
- All mouse/time handlers account for lead-in margin (ruler, clip
drag, section drag/resize, context menu, double-click)
- formatBeat returns 0-indexed bar
- Sub-tab ruler has leadInMargin=0 (no lead-in in clip editor)
3dtours