feat(studio): timeline inline expansion + __clipTree runtime primitive

When a child element inside a sub-composition is selected, the timeline
replaces the parent scene clip with the deepest-level siblings. Deselect
or selecting outside collapses back. Expanded clips are fully editable —
move, resize, delete, and split — addressed by their real DOM id with
timeline time rebased onto the sub-comp they live in.

Runtime:
- New window.__clipTree API: a read-only hierarchical ClipNode tree
  (id/parentId/children + backing element) so Studio can derive
  parent/child relationships for inline expansion.

Studio:
- useExpandedTimelineElements derives the expanded view from
  selectedElementId + clipParentMap (pure useMemo, no useEffect).
  Each child rebases onto its immediate sub-comp host (start +
  sourceFile), so multi-level nesting targets the right file.
- NLELayout routes expanded-clip edits through the same handlers
  top-level clips use, in local coordinates — edits save to the
  sub-comp source and reflect via reloadPreview (no separate DOM-patch
  path). This is the canonical update; there is no reactive observer.
- findMatchingTimelineElementId resolves sub-comp children with no
  top-level element to `sourceFile#id`.
- Razor tool enabled by default; studio_razor_split analytics event
  fired on single and split-all.
- O(n²) isElementGsapTargeted extracted to gsapTargetCache.ts with a
  cached Set+WeakSet O(1) lookup.

packages/studio/src/hooks/gsapTargetCache.ts

@@ -0,0 +1,65 @@
+import { STUDIO_GSAP_DRAG_INTERCEPT_ENABLED } from "../components/editor/manualEditingAvailability";
+
+type TimelineLike = { getChildren?: (nested: boolean) => Array<{ targets?: () => Element[] }> };
+
+let _gsapCachedTimelines: Record<string, TimelineLike> | undefined;
+let _gsapTargetIds: Set<string> | undefined;
+let _gsapTargetNodes: WeakSet<Element> | undefined;
+
+function addTargetsFromTimeline(tl: TimelineLike, ids: Set<string>, nodes: WeakSet<Element>): void {
+  const children = tl.getChildren?.(true);
+  if (!children) return;
+  for (const child of children) {
+    const targets = child.targets?.();
+    if (!targets) continue;
+    for (const t of targets) {
+      nodes.add(t);
+      if (t.id) ids.add(t.id);
+    }
+  }
+}
+
+function collectGsapTargets(timelines: Record<string, TimelineLike>): {
+  ids: Set<string>;
+  nodes: WeakSet<Element>;
+} {
+  const ids = new Set<string>();
+  const nodes = new WeakSet<Element>();
+  for (const tl of Object.values(timelines)) {
+    if (!tl) continue;
+    try {
+      addTargetsFromTimeline(tl, ids, nodes);
+    } catch {
+      /* teardown race */
+    }
+  }
+  return { ids, nodes };
+}
+
+function readTimelines(iframe: HTMLIFrameElement | null): Record<string, TimelineLike> | undefined {
+  if (!iframe?.contentWindow) return undefined;
+  try {
+    return (iframe.contentWindow as Window & { __timelines?: Record<string, TimelineLike> })
+      .__timelines;
+  } catch {
+    return undefined;
+  }
+}
+
+export function isElementGsapTargeted(
+  iframe: HTMLIFrameElement | null,
+  element: HTMLElement,
+): boolean {
+  if (!STUDIO_GSAP_DRAG_INTERCEPT_ENABLED) return false;
+  const timelines = readTimelines(iframe);
+  if (!timelines) return false;
+
+  if (timelines !== _gsapCachedTimelines) {
+    const cache = collectGsapTargets(timelines);
+    _gsapTargetIds = cache.ids;
+    _gsapTargetNodes = cache.nodes;
+    _gsapCachedTimelines = timelines;
+  }
+
+  return _gsapTargetNodes!.has(element) || !!(element.id && _gsapTargetIds!.has(element.id));
+}