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(

packages/studio/src/components/editor/domEditingLayers.test.ts

@@ -0,0 +1,30 @@
+// @vitest-environment jsdom
+import { describe, expect, it } from "vitest";
+import { resolveDomEditSelection } from "./domEditingLayers";
+
+const opts = { activeCompositionPath: "index.html", isMasterView: true, skipSourceProbe: true };
+
+describe("resolveDomEditSelection — hfId from data-hf-id", () => {
+  it("populates hfId from the element data-hf-id attribute", async () => {
+    const el = document.createElement("div");
+    el.id = "hero";
+    el.setAttribute("data-hf-id", "hf-x7k2");
+    document.body.appendChild(el);
+
+    const selection = await resolveDomEditSelection(el, opts);
+    document.body.removeChild(el);
+
+    expect(selection?.hfId).toBe("hf-x7k2");
+  });
+
+  it("leaves hfId undefined when element has no data-hf-id", async () => {
+    const el = document.createElement("div");
+    el.id = "no-hfid-el";
+    document.body.appendChild(el);
+
+    const selection = await resolveDomEditSelection(el, opts);
+    document.body.removeChild(el);
+
+    expect(selection?.hfId).toBeUndefined();
+  });
+});

packages/studio/src/components/editor/domEditingLayers.ts

@@ -369,6 +369,7 @@ export async function resolveDomEditSelection(
     return {
       element: current,
       id: current.id || undefined,
+      hfId: current.getAttribute("data-hf-id") ?? undefined,
       selector,
       selectorIndex,
       sourceFile,