Re-architect the timeline row model from data-track-index rows to stacking
layers. Rows represent stacking layers per context: explicit-z clips merge onto
one track when they share a z and don't overlap in time; auto-z clips stay one
row each (DOM order); audio is pulled into its own bottom lanes. Rows keyed by a
stable layer id, not data-track-index.
Vertical drag always writes z-index, never track: drop onto a layer joins it
(same z), between layers interpolates a new z, past the ends creates a new
front/back layer. data-track-index is never rewritten; #958 holds. Adds
hasExplicitZIndex capture (computed z != auto).
L1: hasExplicitZIndex on the element model
L2: buildStackingTimelineLayers (layer-based rows)
L3: layer-aware vertical drag (join / interpolate / new-extreme)
Miguel Angel Simon Sierra
e5ed512529