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/core/src/runtime/window.d.ts

@@ -1,6 +1,7 @@
 import type { RuntimeTimelineMessage, RuntimeTimelineLike } from "./types";
 import type { HyperframePickerApi } from "../inline-scripts/pickerApi";
 import type { PlayerAPI } from "../core.types";
+import type { ClipTree } from "./clipTree";
 
 type ThreeClockLike = {
   elapsedTime: number;
@@ -29,6 +30,7 @@ declare global {
     __timelines: Record<string, RuntimeTimelineLike>;
     __player?: PlayerAPI;
     __clipManifest?: RuntimeTimelineMessage;
+    __clipTree?: ClipTree;
     __playerReady?: boolean;
     __renderReady?: boolean;
     __hfRuntimeTeardown?: (() => void) | null;