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/App.tsx

@@ -11,6 +11,7 @@ import { usePanelLayout } from "./hooks/usePanelLayout";
 import { useFileManager } from "./hooks/useFileManager";
 import { usePreviewPersistence } from "./hooks/usePreviewPersistence";
 import { useTimelineEditing } from "./hooks/useTimelineEditing";
+import type { TimelineZIndexReorderCommit } from "./hooks/useTimelineEditingTypes";
 import type { BlockPreviewInfo } from "./components/sidebar/BlocksTab";
 import { useDomEditSession } from "./hooks/useDomEditSession";
 import { useSdkSession } from "./hooks/useSdkSession";
@@ -19,7 +20,7 @@ import { useBlockHandlers } from "./hooks/useBlockHandlers";
 import { useAppHotkeys } from "./hooks/useAppHotkeys";
 import { useClipboard } from "./hooks/useClipboard";
 import { readStudioUiPreferences, writeStudioUiPreferences } from "./utils/studioUiPreferences";
-import { selectedKeyframePercentagesForElement } from "./utils/keyframeSelection";
+import { deleteSelectedKeyframes } from "./hooks/timelineEditingHelpers";
 import { useCaptionDetection } from "./hooks/useCaptionDetection";
 import { useRenderClipContent } from "./hooks/useRenderClipContent";
 import { useConsoleErrorCapture } from "./hooks/useConsoleErrorCapture";
@@ -140,6 +141,7 @@ export function StudioApp() {
   });
   const editHistory = usePersistentEditHistory({ projectId });
   const domEditSaveTimestampRef = useRef(0);
+  const handleDomZIndexReorderCommitRef = useRef<TimelineZIndexReorderCommit | null>(null);
   const pendingTimelineEditPathRef = useRef(new Set<string>());
   const isGestureRecordingRef = useRef(false);
   const reloadPreview = useCallback(() => setRefreshKey((k) => k + 1), []);
@@ -188,6 +190,7 @@ export function StudioApp() {
     isRecordingRef: isGestureRecordingRef,
     sdkSession: sdkHandle.session,
     forceReloadSdkSession: sdkHandle.forceReload,
+    handleDomZIndexReorderCommitRef,
   });
   const {
     activeBlockParams,
@@ -306,19 +309,12 @@ export function StudioApp() {
     forceReloadSdkSession: sdkHandle.forceReload,
   });
   domEditSelectionBridgeRef.current = domEditSession.domEditSelection;
+  handleDomZIndexReorderCommitRef.current = domEditSession.handleDomZIndexReorderCommit;
   clearDomSelectionRef.current = domEditSession.clearDomSelection;
   handleDomEditElementDeleteRef.current = domEditSession.handleDomEditElementDelete;
   resetKeyframesRef.current = domEditSession.handleResetSelectedElementKeyframes;
   invalidateGsapCacheRef.current = domEditSession.invalidateGsapCache;
-  deleteSelectedKeyframesRef.current = () => {
-    const { selectedKeyframes, selectedElementId } = usePlayerStore.getState();
-    const a = domEditSession.selectedGsapAnimations.find((x) => x.keyframes);
-    if (!a) return;
-    // Only the active element's keyframes; a stale cross-element selection must not delete here.
-    for (const p of selectedKeyframePercentagesForElement(selectedKeyframes, selectedElementId)) {
-      domEditSession.handleGsapRemoveKeyframe(a.id, p);
-    }
-  };
+  deleteSelectedKeyframesRef.current = () => deleteSelectedKeyframes(domEditSession);
   useSdkSelectionSync(
     sdkHandle.session,
     domEditSession.domEditSelection,