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/components/editor/domEditingElement.ts

@@ -95,7 +95,7 @@ function isInspectableLayerElement(el: HTMLElement): boolean {
 export function getDomLayerPatchTarget(
   el: HTMLElement,
   activeCompositionPath: string | null,
-): Pick<DomEditSelection, "id" | "selector" | "selectorIndex" | "sourceFile"> | null {
+): Pick<DomEditSelection, "id" | "hfId" | "selector" | "selectorIndex" | "sourceFile"> | null {
   if (!isInspectableLayerElement(el)) return null;
   if (el.hasAttribute("data-composition-id")) return null;
 
@@ -105,6 +105,7 @@ export function getDomLayerPatchTarget(
   const { sourceFile } = getSourceFileForElement(el, activeCompositionPath);
   return {
     id: el.id || undefined,
+    hfId: el.getAttribute("data-hf-id") || undefined,
     selector,
     selectorIndex: getSelectorIndex(
       el.ownerDocument,
@@ -229,9 +230,14 @@ export function isLargeRasterDomEditSelection(
 
 export function findElementForSelection(
   doc: Document,
-  selection: Pick<DomEditSelection, "id" | "selector" | "selectorIndex" | "sourceFile">,
+  selection: Pick<DomEditSelection, "id" | "hfId" | "selector" | "selectorIndex" | "sourceFile">,
   activeCompositionPath: string | null = null,
 ): HTMLElement | null {
+  if (selection.hfId) {
+    const byHfId = doc.querySelector(`[data-hf-id="${selection.hfId}"]`);
+    if (isHtmlElement(byHfId)) return byHfId;
+  }
+
   if (selection.id) {
     const byId = doc.getElementById(selection.id);
     if (