feat(studio): timeline collision and placement model

What: new pure module timelineCollision — zone-aware drop placement
(clampTrackToZone, resolveZoneDropPlacement, resolveInsertRow,
resolvePlacement, lane/overlap predicates) with its full test suite.

Why: the no-overlap core of the NLE clip-drag engine; plain functions, no
DOM, no React, no store writes.

How: new files only; type-only imports from the existing playerStore.
First runtime consumer arrives with the drag-engine PRs.

Test plan: bunx vitest run timelineCollision.test.ts; tsc --noEmit; fallow
audit clean (all exports test-consumed).

Author: ukimsanov