L4 polish for the stacking-layer timeline:
- Sub-composition layers group under a slim "Inside: <comp>" header with a green
accent, so it reads that restacking there is scoped to that context.
- Audio clips render in a distinct bottom lane: separator border, muted row bg,
music glyph in the gutter, beat strip — clearly not part of the z-order stack.
- During a vertical drag, a drop affordance shows join-vs-insert: an "onto"
target highlights the row (join that layer / same z); "between"/"above"/
"below" shows an insertion line (new layer here).
New pure helper timelineDropIndicator.ts (+ test) maps placement -> indicator;
row-group rendering extracted into TimelineLayerGroupHeader / TimelineLayerGutter
/ TimelineDropInsertionLine / TimelineDragGhost to keep files under the 600 cap.
