fix(studio): make vertical lane moves persist correctly and harden the z/lane pipeline

Vertical clip moves committed in the store but never survived: two persist
bugs plus a runtime renumber all fought the stable-track-lanes model.

- timelineMoveAdapter deliberately stripped the track from lane-reorder
  persists ('z-only reorder path' — the old z-driven lane model). Lane =
  authored data-track-index now: lane-reorder and track-insert both persist
  the track; plain timing moves omit it to stay SDK-fast-path eligible.
- Display lanes and file tracks are different coordinate spaces:
  normalizeToZones packs sparse authored tracks (1,2,... or gaps, or DOM-index
  fallbacks) onto contiguous display lanes, and lane edits persisted the LANE
  number — silently re-targeting the wrong row in any non-0-contiguous file.
  Elements now record their authoredTrack when remapped; a lane change
  persists the target lane's authored track (store stays in lane space).
- The runtime split same-track clips of different kinds (video vs caption
  div) onto separate renumbered tracks at discovery, so authored indices
  never round-tripped ('drop onto an existing track' bounced back). Removed:
  data-track-index is honored verbatim (render never reads it); kind-based
  row presentation belongs in the display layer if ever wanted.

Adversarial review fixes on the same pipeline:
- runtime: parseInt(attr) || fallback dropped authored track 0 for GSAP and
  overlay clips (parseAuthoredTrack helper honors 0)
- single-clip move fallback persisted only data-start — lane changes snapped
  back on reload (now passes the track to the patch builder)
- lane-change z-sync candidate ignored a multi-selection's time shift, so
  patches were computed against stale overlap sets
- track insert around a locked clip persisted a colliding renumber (the next
  normalize merged lanes); the insert is now refused with a warning
- computeStackingPatches compared leaf z across CSS stacking contexts, where
  ancestor z decides paint order; the sync now partitions by
  stackingContextId and never patches across contexts

Timeline geometry (user-reported):
- fit zoom leaves 20% trailing headroom (FIT_ZOOM_HEADROOM in
  timelineLayout.ts; single fit-pps source, so ruler/lanes/playhead/drag all
  inherit it)
- playhead line center now sits exactly on GUTTER + t*pps at every zoom
  (wrapper had shrink-wrapped to the 9px diamond, off-centering the line);
  ruler ticks center on their timestamp
- ruler: frame-mode steps snap to whole frames (no duplicate labels), hour
  steps added for far zoom-out, tick positions computed as exact multiples
  (no float drift)

packages/studio/src/hooks/useTimelineGroupEditing.ts

@@ -77,6 +77,15 @@ function resizeCoalesceKey(changes: readonly TimelineGroupResizeChange[]): strin
   return `timeline-group-resize:${changes.map((change) => change.element.hfId ?? change.element.id).join(",")}`;
 }
 
+function toSdkTimingChanges<T extends { element: TimelineElement }>(
+  changes: readonly T[],
+  timingUpdate: (change: T) => { start: number; duration?: number },
+): Array<{ hfId: string; timingUpdate: { start: number; duration?: number } } | null> {
+  return changes.map((change) =>
+    change.element.hfId ? { hfId: change.element.hfId, timingUpdate: timingUpdate(change) } : null,
+  );
+}
+
 function resizeHasPlaybackStartAdjustment(change: TimelineGroupResizeChange): boolean {
   return (
     change.playbackStart != null ||
@@ -225,11 +234,7 @@ export function useTimelineGroupEditing({
         await options?.beforeTiming;
         const handledBySdk = await trySdkBatchPersist({
           changes,
-          sdkChanges: changes.map((change) =>
-            change.element.hfId
-              ? { hfId: change.element.hfId, timingUpdate: { start: change.start } }
-              : null,
-          ),
+          sdkChanges: toSdkTimingChanges(changes, (change) => ({ start: change.start })),
           eligible: changes.every((change) => change.track == null),
           needsExtension,
           label: "Move timeline clips",
@@ -314,14 +319,10 @@ export function useTimelineGroupEditing({
         await options?.beforeTiming;
         const handledBySdk = await trySdkBatchPersist({
           changes,
-          sdkChanges: changes.map((change) =>
-            change.element.hfId
-              ? {
-                  hfId: change.element.hfId,
-                  timingUpdate: { start: change.start, duration: change.duration },
-                }
-              : null,
-          ),
+          sdkChanges: toSdkTimingChanges(changes, (change) => ({
+            start: change.start,
+            duration: change.duration,
+          })),
           eligible: changes.every((change) => !resizeHasPlaybackStartAdjustment(change)),
           needsExtension,
           label: "Resize timeline clips",