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/useElementLifecycleOps.test.tsx

@@ -18,6 +18,7 @@ afterEach(() => {
 interface BatchOptions {
   label: string;
   coalesceKey: string;
+  skipReload?: boolean;
 }
 
 interface CapturedBatchCall {
@@ -110,6 +111,25 @@ describe("useElementLifecycleOps — z-index reorder payload", () => {
     act(() => root.unmount());
   });
 
+  it("requests skipReload on every z-reorder persist (live DOM already final)", async () => {
+    // The commit applies the z-index (and any injected position) to the live
+    // iframe DOM and the store synchronously, so the persisted style-only patch
+    // adds nothing the preview doesn't already show — the batch commit is asked
+    // to skip the iframe remount. commitDomEditPatchBatches still falls back to
+    // reloading when the server can't confirm every patch target matched.
+    const el = document.createElement("div");
+    el.id = "clip-z";
+
+    const { captured, root } = await runReorderCommit(el, [
+      { element: el, zIndex: 4, id: "clip-z", sourceFile: "index.html" },
+    ]);
+
+    expect(captured).toHaveLength(1);
+    expect(captured[0]?.options.skipReload).toBe(true);
+
+    act(() => root.unmount());
+  });
+
   it("preserves a real id when the element has one", async () => {
     const el = document.createElement("video");
     el.id = "v-hero";