fix(studio): harden composition timeline reliability (#2615)

* fix(studio): preserve composition playback continuity

* feat(studio): drag compositions into the timeline

* fix(studio): collapse expanded composition move aliases

* fix(studio): make timeline cuts atomic

* fix(studio): group inspector gesture history

* test(studio): cover masked text selection

* fix(studio): harden composition timeline reliability

* fix(studio): satisfy CI source gates

* fix(studio): harden composition mutation requests

packages/studio/src/hooks/useTimelineGroupEditing.ts

@@ -12,6 +12,7 @@ import {
   extendRootDurationIfNeeded,
   formatTimelineAttributeNumber,
   patchIframeDomTiming,
+  playbackStartAttributeForElement,
   persistTimelineBatchEdit,
   type PersistTimelineBatchChange,
   type RecordEditInput,
@@ -242,9 +243,10 @@ export function useTimelineGroupEditing({
       // duration change), so nothing timing-related changed — the batch only
       // rewrites data-track-index, which the renderer never reads (documented
       // in core runtime/timeline.ts; track is a studio lane concept). The live
-      // DOM patch above + the gesture owner's optimistic store update fully
-      // cover the UI, so after the persist there is nothing to GSAP-shift and
-      // nothing for the preview to recompute: skip the fallback below entirely.
+      // DOM patch above + the gesture owner's optimistic store update cover the
+      // in-flight UI; after the complete lane + z transaction, that owner
+      // refreshes the preview so its runtime manifest converges to disk. There
+      // is still nothing to GSAP-shift here, so skip this fallback entirely.
       // Running it anyway is what made the mirrored z-order lane move blink —
       // a zero-delta batch yields no scriptText, and finishGroupTimingGsapFallback
       // used to full-reload the iframe when there was no script to soft-swap
@@ -348,10 +350,7 @@ export function useTimelineGroupEditing({
           ["data-duration", formatTimelineAttributeNumber(change.duration)],
         ];
         if (change.playbackStart != null) {
-          const liveAttr =
-            change.element.playbackStartAttr === "playback-start"
-              ? "data-playback-start"
-              : "data-media-start";
+          const liveAttr = playbackStartAttributeForElement(change.element);
           liveAttrs.push([liveAttr, formatTimelineAttributeNumber(change.playbackStart)]);
         }
         patchIframeDomTiming(previewIframeRef.current, change.element, liveAttrs, activeCompPath);