fix(studio): keep timeline selection authoritative in the preview sync

The store-to-preview sync no longer applies a partial selection: if a resolvable member's
DOM node is not ready yet it bails and retries on the next effect run, so the write-back can
never shrink the store's selection by dropping an unresolved member.

Marquee row hit-testing reuses shouldShowTimelineLayerGroupHeader instead of re-deriving
the group-header placement rule, keeping one owner for that predicate.

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

@@ -2,6 +2,7 @@ import { formatTime } from "../lib/time";
 import { roundToCenti } from "../../utils/rounding";
 import type { StackingTimelineLayer, TimelineLayerId } from "./timelineTrackOrder";
 import { resolveTimelineLayerStackingMove } from "./timelineLayerDrag";
+import { shouldShowTimelineLayerGroupHeader } from "./TimelineLayerGroupHeader";
 import type { TimelineStackingElement, TimelineStackingReorderIntent } from "./timelineStacking";
 
 import {
@@ -348,7 +349,7 @@ export function selectTimelineElementsInMarquee({
   for (const layerId of layerOrder) {
     const layer = layerById.get(layerId);
     if (!layer) continue;
-    if (layer.contextKey !== "" && layer.contextKey !== previousContextKey) {
+    if (shouldShowTimelineLayerGroupHeader(layer.contextKey, previousContextKey)) {
       rowTop += groupHeaderHeight;
     }
     const rowBottom = rowTop + trackHeight;