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/useDomGeometryCommits.ts

@@ -1,5 +1,4 @@
 import { useCallback } from "react";
-import { STUDIO_GSAP_DRAG_INTERCEPT_ENABLED } from "../components/editor/manualEditingAvailability";
 import { getDomEditTargetKey, type DomEditSelection } from "../components/editor/domEditing";
 import {
   applyStudioPathOffset,
@@ -19,45 +18,11 @@ import {
 } from "../components/editor/manualEditsDomPatches";
 import type { DomEditGroupPathOffsetCommit } from "../components/editor/DomEditOverlay";
 import type { PatchOperation } from "../utils/sourcePatcher";
+import { isElementGsapTargeted } from "./gsapTargetCache";
 
 export const GSAP_CSS_FALLBACK_BLOCKED_MESSAGE =
   "This element is GSAP-animated — dragging via CSS would corrupt keyframes";
 
-// ── Helpers ──
-
-type TimelineLike = { getChildren?: (nested: boolean) => Array<{ targets?: () => Element[] }> };
-
-// fallow-ignore-next-line complexity
-function isElementGsapTargeted(iframe: HTMLIFrameElement | null, element: HTMLElement): boolean {
-  // When the GSAP drag intercept is disabled for debugging, treat every
-  // element as un-targeted so commits take the plain CSS persist path.
-  if (!STUDIO_GSAP_DRAG_INTERCEPT_ENABLED) return false;
-  if (!iframe?.contentWindow) return false;
-  let timelines: Record<string, TimelineLike> | undefined;
-  try {
-    timelines = (iframe.contentWindow as Window & { __timelines?: Record<string, TimelineLike> })
-      .__timelines;
-  } catch {
-    return false;
-  }
-  if (!timelines) return false;
-  const id = element.id;
-  for (const tl of Object.values(timelines)) {
-    if (!tl?.getChildren) continue;
-    try {
-      for (const child of tl.getChildren(true)) {
-        if (!child.targets) continue;
-        for (const t of child.targets()) {
-          if (t === element || (id && t.id === id)) return true;
-        }
-      }
-    } catch {
-      continue;
-    }
-  }
-  return false;
-}
-
 // ── Hook ──
 
 interface UseDomGeometryCommitsParams {