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/useDomEditCommits.ts

@@ -14,6 +14,7 @@ import { useDomEditPositionPatchCommit } from "./useDomEditPositionPatchCommit";
 import { useDomEditTextCommits } from "./useDomEditTextCommits";
 import { useDomGeometryCommits } from "./useDomGeometryCommits";
 import { useElementLifecycleOps } from "./useElementLifecycleOps";
+import { formatFieldsSuffix } from "./gsapScriptCommitHelpers";
 
 // ── Helpers ──
 
@@ -31,14 +32,7 @@ async function readErrorResponseBody(
 
 function formatPatchRejectionMessage(body: { error?: string; fields?: string[] } | null): string {
   if (!body?.error) return "Couldn't save edit";
-  // Pre-existing clone of the GSAP save-error formatter (gsapScriptCommitHelpers);
-  // surfaced here by this PR's adjacent edits, not introduced by it.
-  // fallow-ignore-next-line code-duplication
-  const fields = Array.isArray(body.fields)
-    ? body.fields.filter((field): field is string => typeof field === "string")
-    : [];
-  const suffix = fields.length > 0 ? ` (${fields.join(", ")})` : "";
-  return `Couldn't save edit: ${body.error}${suffix}`;
+  return `Couldn't save edit: ${body.error}${formatFieldsSuffix(body.fields)}`;
 }
 
 interface RecordEditInput {