feat(timeline-editor): Diagonal scroll following time-ordered sequence cascade

Enhancement: Smart Vertical Scrolling
- Mouse wheel now scrolls diagonally (horizontal + vertical)
- Automatically brings relevant sequence to top-left of viewport
- Follows the time-ordered cascade of sequences

Algorithm:
1. Scroll horizontally as before (timeline.scrollLeft += deltaY)
2. Calculate current time position (left edge of viewport)
   currentTime = scrollLeft / pixelsPerSecond
3. Find closest sequence at that time:
   - Last sequence that starts before or at currentTime
   - Sequences sorted by startTime
4. Smooth scroll vertically to target sequence:
   - targetScrollTop = seqIndex * 80px
   - Smooth transition: scrollTop += diff * 0.3 (not instant jump)

User Experience:
- Scroll forward in time → later sequences move to top
- Scroll backward in time → earlier sequences move to top
- Creates natural "diagonal" navigation along timeline
- Top-left corner always shows the most relevant sequence

Example timeline:
  Seq 0 (0s)  ──────────────
  Seq 1 (5s)         ──────────────
  Seq 2 (10s)                ──────────────

Scroll to 7s → Seq 1 moves to top (it's active at that time)
Scroll to 12s → Seq 2 moves to top (it's active at that time)

This provides intuitive navigation through time-ordered sequences.

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