feat(studio,core): populate hfId in DomEditSelection + widen MutationTarget (R7, T5a) (#1296)

## Summary

- Adds `hfId` field to `resolveDomEditSelection` — reads `data-hf-id` off the live element and stores it in `DomEditSelection.hfId`
- `DomEditSelection extends PatchTarget` which already declares `hfId?: string`, so this is a single new line at the return site
- Widens `MutationTarget` in `files.ts` to include `hfId?: string` (type hygiene — the value already survives through `parseMutationBody`'s by-reference pass, so this is documentation not a behaviour change)

## Why

R7 / Task 5a. The full hf-id write-back and patch-engine infrastructure (R1 + R7 Tasks 0–4, PRs #1269–#1292) is server-complete. The only missing piece was: the Studio client never read `data-hf-id` off a hit-tested element, so `target.hfId` was always `undefined` and the `hfId`-first lookup branches in both patch engines were unreachable in production. This PR fixes the selection side — the commit wire (#1297) completes the path.

## Test plan

- [ ] `packages/studio/src/components/editor/domEditingLayers.test.ts` — two new tests with jsdom environment:
  - `resolveDomEditSelection` on an element with `data-hf-id` → `selection.hfId` is populated
  - element without `data-hf-id` → `selection.hfId` is `undefined`
- [ ] All 65 studio test files pass, all 72 core test files pass

🤖 Generated with [Claude Code](https://claude.com/claude-code)

packages/core/src/studio-api/routes/files.ts

@@ -84,7 +84,12 @@ function resolveFileMutationContext(c: RouteContext, adapter: StudioApiAdapter,
   return resolveProjectPath(c, adapter, (id) => `/projects/${id}/file-mutations/${operation}/`);
 }
 
-type MutationTarget = { id?: string | null; selector?: string; selectorIndex?: number };
+type MutationTarget = {
+  id?: string | null;
+  hfId?: string;
+  selector?: string;
+  selectorIndex?: number;
+};
 
 /** Write `next` to `absPath` only if it differs from `original`, returning a standardized change response. */
 function writeIfChanged(