fix(studio): flashless z-order commits and visible-overlap stepping

Two legibility fixes for the canvas z-order menu, from user feel-testing:

- z-only commits no longer remount the preview iframe. The commit hook
  already applies the inline z (+ injected position) to the live elements and
  updates the store synchronously; the post-commit reloadPreview() was a
  redundant full remount that read as a canvas 'blink' on every action.
  commitDomEditPatchBatches gains skipReload, engaged only when provably
  safe: every op is an inline-style patch AND the server reports every patch
  matched — anything else falls back to the reload so the preview reconverges
  with disk. The file-watcher's own reload stays suppressed by the existing
  domEditSaveTimestampRef window, so the skip is real.
- Bring Forward / Send Backward step over the next VISIBLY overlapping
  sibling. The nearest z-neighbor in a composition is often invisible at the
  current frame (runtime hides time-inactive clips with inline
  visibility/display; GSAP parks elements at opacity 0), so the step crossed
  something the user couldn't see — 'enabled but nothing happens'. The
  forward/backward set now filters on element-level computed visibility
  (display/visibility/opacity, injectable for tests); enable/disable shares
  the resolver so the menu is honest: actions disable when no visible
  neighbor exists. Front/back keep the full painting family.
- The neighbor that was stepped over gets a 600ms accent flash, drawn in the
  studio overlay layer (never in the iframe DOM), so the action shows its
  work.

packages/studio/src/hooks/useDomEditCommits.test.tsx

@@ -265,7 +265,7 @@ describe("useDomEditCommits z-index reorder persistence", () => {
     document.body.replaceChildren();
   });
 
-  it("persists an N-element reorder with one batch POST, one undo entry, and one reload", async () => {
+  it("persists an N-element reorder with one batch POST, one undo entry, and NO iframe reload", async () => {
     const original =
       '<div id="a" style="z-index: 1"></div><div id="b" style="z-index: 2"></div><div id="c" style="z-index: 3"></div>';
     const after =
@@ -338,6 +338,41 @@ describe("useDomEditCommits z-index reorder persistence", () => {
         coalesceKey: "z-reorder:test",
         files: { "index.html": { before: original, after } },
       });
+      // FIX: a z-only reorder must NOT remount the preview iframe ("the blink").
+      // The live DOM + store already hold the final state and the server matched
+      // every style-only patch, so the reload is provably redundant.
+      expect(rendered.reloadPreview).not.toHaveBeenCalled();
+    } finally {
+      rendered.cleanup();
+    }
+  });
+
+  it("falls back to reloading when the server response omits matched[]", async () => {
+    // Without a matched[] confirmation the persist can't be proven in sync with
+    // the live DOM — the skip-reload path must not engage.
+    const original = '<div id="a" style="z-index: 1"></div>';
+    const after = '<div id="a" style="z-index: 2"></div>';
+    const fetchMock = vi.fn(async (input: Parameters<typeof fetch>[0]): Promise<Response> => {
+      const url = requestUrl(input);
+      if (url.includes("/api/projects/p1/files/")) return jsonResponse({ content: original });
+      if (url.includes("/file-mutations/patch-elements-batch/")) {
+        return jsonResponse({ ok: true, changed: true, content: after });
+      }
+      throw new Error(`Unexpected fetch: ${url}`);
+    });
+    vi.stubGlobal("fetch", fetchMock);
+    const { iframe, element } = createPreviewElement();
+    element.id = "a";
+    const rendered = renderDomEditCommits(createSelection(element), iframe);
+
+    try {
+      await act(async () => {
+        await rendered.hook.handleDomZIndexReorderCommit([
+          { element, zIndex: 2, id: "a", sourceFile: "index.html" },
+        ]);
+      });
+
+      expect(rendered.recordEdit).toHaveBeenCalledTimes(1);
       expect(rendered.reloadPreview).toHaveBeenCalledTimes(1);
     } finally {
       rendered.cleanup();
@@ -347,7 +382,9 @@ describe("useDomEditCommits z-index reorder persistence", () => {
   it("warns and reports telemetry for unmatched batch patches without throwing", async () => {
     // The server reports per-patch matched[]: #b was not found in the source.
     // The matched subset persisted, so the commit must complete (no rollback of
-    // applied state) while surfacing the partial failure.
+    // applied state) while surfacing the partial failure. An unmatched target
+    // also means the live DOM shows z-order the disk lacks, so the skip-reload
+    // path must NOT engage — the reload reconverges the preview with disk.
     const original = '<div id="a" style="z-index: 1"></div>';
     const after = '<div id="a" style="z-index: 2"></div>';
     const fetchMock = vi.fn(async (input: Parameters<typeof fetch>[0]): Promise<Response> => {