fix(studio): persist canvas z-order actions correctly for static elements

An adversarial review of the canvas context-menu z-order pipeline (Bring to
Front / Forward / Backward / Send to Back) found the resolver math sound but
the glue between the menu and the commit hook broken:

- The menu optimistically wrote style.zIndex AND position: relative to the
  live elements BEFORE the commit hook ran. The hook decides whether to
  persist position by checking getComputedStyle(el).position === 'static' —
  always false after the pre-apply — so the position patch was never
  persisted on the menu path and the reorder silently reverted at the
  post-commit reload for any nested/static element (root clips survive only
  because the runtime forces position:absolute). The same pre-apply made the
  failure rollback capture the already-mutated values, restoring the broken
  state on persist errors. The menu no longer pre-applies; the hook owns the
  live writes (it already applied both synchronously) and now sees true
  priors. Siblings without a persistable identity still get their z applied
  live-only so a renumber stays visually coherent.
- The commit hook's entry.key store-sync plumbing had zero production
  callers; the store zIndex went stale until full reload. All three callers
  (canvas menu via PreviewOverlays, timeline lane z-sync, LayersPanel) now
  derive and pass the timeline store key (new deriveTimelineStoreKey helper).
- patchElementBatch discarded the server's per-patch matched[]; unresolvable
  siblings persisted partially and silently. Unmatched targets now warn and
  report save-failure telemetry (z-reorder-unmatched) without rolling back
  the matched subset.
- template/noscript elements counted as painting siblings, so renumber
  fallbacks wrote z-index/position into <template> tags in the source file.
  Excluded from the sibling family.
- The default undo coalesce key merged DISTINCT z actions within 300ms into
  one undo entry; the action kind is now part of the key (LayersPanel drags
  keep coalescing within a drag; explicit lane-move gesture keys untouched).
- rectsIntersect comment claimed touching rects intersect; the strict
  inequalities say otherwise — comment fixed.

packages/studio/src/components/editor/LayersPanel.tsx

@@ -15,6 +15,7 @@ import {
 import { Layers } from "../../icons/SystemIcons";
 import { useLayerDrag, isLayerDraggable, type LayerReorderEvent } from "./useLayerDrag";
 import { computeReorderZValues, getElementZIndex } from "../../player/lib/layerOrdering";
+import { deriveTimelineStoreKey } from "../../player/lib/timelineElementHelpers";
 
 const TAG_ICONS: Record<string, string> = {
   video: "Vi",
@@ -280,9 +281,17 @@ export const LayersPanel = memo(function LayersPanel() {
         selector: layer.selector,
         selectorIndex: layer.selectorIndex,
         sourceFile: layer.sourceFile,
+        key: deriveTimelineStoreKey({
+          domId: layer.id,
+          selector: layer.selector,
+          selectorIndex: layer.selectorIndex,
+          sourceFile: layer.sourceFile,
+        }),
       }));
 
-      handleDomZIndexReorderCommit(entries);
+      // "layer-drag" keeps consecutive drops of the same sibling set coalescing
+      // into one undo step, without merging with a context-menu z action.
+      handleDomZIndexReorderCommit(entries, undefined, "layer-drag");
     },
     [handleDomZIndexReorderCommit],
   );