feat(studio): unify timeline vertical reorder with z-index stacking

Timeline rows now order by scoped stacking (z-index per stacking context)
instead of data-track-index, and dragging a clip up/down commits a targeted
z-index change through the same shared path the layers panel uses. Both panels
stay consistent and moving a clip actually changes front/back. data-track-index
is demoted to time-overlap layout only; no bulk z-index injection (#958 intact).

Also restores beat-snapping on keyframe retiming (re-wires snapKeyframePctToBeat,
orphaned when keyframe dragging was removed) which surfaced while unifying the
model. Extracts pure track-ordering logic to timelineTrackOrder.ts, the stacking
reorder commit + deleteSelectedKeyframes to timelineEditingHelpers.ts, to keep
StudioApp / the timeline hook / Timeline under the studio 600-LOC cap.

U3: scoped stacking row order in the timeline
U4: vertical drag commits z-index via the shared reorder commit
U8: restore keyframe beat-snap on retime

packages/studio/src/player/components/timelineCallbacks.ts

@@ -1,7 +1,7 @@
 // fallow-ignore-file code-duplication
 // fallow-ignore-file dead-code
 import type { TimelineElement } from "../store/playerStore";
-import type { BlockedTimelineEditIntent } from "./timelineEditing";
+import type { BlockedTimelineEditIntent, TimelineStackingReorderIntent } from "./timelineEditing";
 
 /**
  * Shared callback signatures for timeline editing operations.
@@ -26,7 +26,9 @@ export interface TimelineDropCallbacks {
 export interface TimelineEditCallbacks {
   onMoveElement?: (
     element: TimelineElement,
-    updates: Pick<TimelineElement, "start" | "track">,
+    updates: Pick<TimelineElement, "start" | "track"> & {
+      stackingReorder?: TimelineStackingReorderIntent | null;
+    },
   ) => Promise<void> | void;
   onResizeElement?: (
     element: TimelineElement,