feat(studio): carry hfId on TimelineElement, wire through buildPatchTarget (R7, T5b) (#1299)

* feat(studio): carry hfId on TimelineElement, wire through buildPatchTarget (R7, T5b)

* refactor(studio): extract readHfId helper, fix empty-string normalization, add comments (R7 review)

- Extract readHfId(el) to domEditingLayers.ts — centralizes `?.trim() || undefined`
  normalisation; guards against empty-string data-hf-id reaching findTagByTarget
- Wire readHfId into domEditingLayers.ts and useDomEditCommits.ts (the one site
  that still used `?? undefined` instead of `|| undefined`)
- Re-export readHfId through domEditing.ts public API
- Add readHfId unit tests: present, absent, empty-string, whitespace-only
- Add comment on PatchTarget: runtime validation lives in findTagByTarget, type is docs-only
- Suppress pre-existing unused re-exports in timelineDOM.ts (backward-compat re-exports
  brought into fallow scope by the T5b hfId changes)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* fix(core): clear data-hf-id on split clone to prevent dual-match (R7 review)

cloneNode(true) copies all attributes including data-hf-id. Without clearing it,
both halves of a split share the same hf-id; the server's findByHfId picks the first
match and silently patches the wrong clip. Remove the attribute from the clone so
write-back re-mints a fresh id on the next preview load.

Adds a test: splitElementInHtml — hfId clone isolation.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* fix(studio): add hfId to DomEditLayerItem + getDomLayerPatchTarget return type (R7 review)

- Add hfId to DomEditLayerItem interface (domEditingTypes.ts) so layer item
  construction in collectDomEditLayerItems compiles
- Widen getDomLayerPatchTarget return type to include hfId + populate it from
  data-hf-id attribute (domEditingElement.ts)
- Widen findDomEditSelectionTarget to check hfId-first when no id/selector
- Widen Pick types in domEditOverlayGeometry.ts and useGsapScriptCommits.ts
- Add hfId to buildMissingCompositionElements element construction
- Add hfId-targeted test coverage in domEditing.test.ts,
  domEditOverlayGeometry.test.ts, timelineIframeHelpers.test.ts
- Update hfIds.test.ts KNOWN LIMITATION labels — write-back landed in R7 T1-2

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/studio/src/utils/sourcePatcher.ts

@@ -92,6 +92,8 @@ export interface PatchOperation {
   value: string | null;
 }
 
+// Runtime validation for hfId lives in findTagByTarget → execDataAttrPattern (CSS attr-value
+// escape). This type is documentation only; the server's MutationTarget mirrors this shape.
 export interface PatchTarget {
   id?: string | null;
   hfId?: string;