feat(studio): timeline track = stacking layer (NLE-style layering)

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)

packages/studio/src/player/hooks/useTimelinePlayer.test.ts

@@ -243,7 +243,10 @@ describe("buildStandaloneRootTimelineElement", () => {
         iframeSrc: "http://127.0.0.1:4173/api/projects/demo/preview/comp/scenes/hero.html?_t=123",
         selector: '[data-composition-id="hero"]',
       }),
-    ).toEqual({
+      // toMatchObject (not toEqual): asserts the selector/source metadata this
+      // test is about, without re-pinning the full element shape (stacking
+      // metadata like hasExplicitZIndex is covered in timelineDOM.test.ts).
+    ).toMatchObject({
       id: "hero",
       label: "hero",
       key: 'scenes/hero.html:[data-composition-id="hero"]:0',