WIP: Iterative buildCells rewrite

What's done

Block has weight uint64, triggers []*Trigger, topRow int fields
TriggerSource and TriggerTarget have block *Block pointer fields
linkTriggers() resolves string IDs to block pointers, populates block.triggers
computeWeights() sets weight = (cue index) << 32 + 0 if cue-ended, 1 otherwise
- Higher weight = more likely to be moved
- Later cues = higher weight
buildCells() is rewritten to be iterative:
1. Init: cues get sequential topRow, non-cues start at 0
2. Loop (trigger pass): move target blocks so trigger source row >= target hook row
3. Loop (overlap pass): sort same-track blocks by (topRow, weight), push overlaps to prev end + 1
4. After convergence: move each cue to max row of its triggered cells
5. emitCells() creates actual TimelineCell objects from computed topRows
Test uses BuildTimelineDebug with testWriter for debug output

Cue track comes out unordered after the cue-move step (cues placed at their highest triggered cell row, which scrambles scene order)
Gap padding in emitCells skips cue track (if block.Type != "cue") so cue cell rows don't match topRow
The +1 gap between blocks may be wrong (was +2, changed to +1)
assignRows still can't converge after buildCells — off-by-one misalignments between cue rows and target rows

topRow coordinate space should include gap cells (emitCells just places cells at topRow)
blockHeight: cue=0 (own track, can overlap), other=4
Gap between same-track blocks = 1 row (chain or gap cell)
Cross-track alignment still handled by assignRows solver after buildCells

Fix cue track: need gap padding on cue track too so cell rows match topRow
May need to rethink cue-move step — moving cues to highest triggered row scrambles cue order
Verify topRow math matches actual emitted cell rows end-to-end