fix(studio): restore golden-branch timeline behaviors dropped by the stack rebuild

The Studio stack rebuild (#2291) landed the remaining NLE layers but dropped
or regressed several final-wave behaviors from the reviewed studio-dnd stack,
and never repaired the stale timelineZones.ts that #2279 introduced. Restores:

- TimelineRuler: sticky under vertical scroll, full-height gridlines removed
  (beat lines only), frame-number tick labels via a persisted timeDisplayMode
  store preference (PlayerControls toggle now store-backed)
- timelineZones: stable track lanes — lane = authored data-track-index
  ascending; z is paint order only (replaces the stale z-driven lane pack,
  which broke track insert-band commits that contractually depend on it)
- persistTimelineBatchEdit: a batch member whose patch is a no-op (attributes
  already at target values, e.g. in a track-insert renumber) is skipped
  instead of aborting and rolling back the whole batch — this alone made
  new-track creation (incl. the top insert band) fail silently
- useTimelineStackingSync: unresolvable clips read as NaN again so
  timelineStackingSync's Number.isFinite exclusion contract holds (z=0
  fabrications skewed stacking boundaries)
- timelineAssetDrop: drops land on the drop track (no overlap bump to
  max-track+1), data-hf-id stamped, audio gets data-volume
- timing edits: soft-reload the server's rewritten GSAP script instead of a
  full iframe remount (no all-clips flash on move/resize); full reload only
  when no scriptText or the soft path can't apply, and one full reload when a
  group edit touches non-active files (new hooks/timelineTimingSync.ts)
- duration: content-driven grow-AND-shrink on move/resize/delete, synced
  optimistically to the store and the live root data-duration at release
  (was a grow-only ratchet; shrink never updated the readout)

New UX: sidebar asset click opens a compact non-modal preview over the canvas
(dismiss on outside click, Escape, playback, or seek), and clicking an
already-added asset reveals its clip in the timeline (smooth minimal scroll
to its time and lane; vertical-only in fit zoom).

Verified by pointer-driving a real project: sticky ruler + gridline removal,
no iframe remount on move/resize (marker survives, GSAP tween positions
rewritten in place), duration readout 40->37->40 on shrink/stretch, and
top-insert-band track creation renumbering lanes correctly on disk.

packages/studio/src/player/components/useTimelineStackingSync.ts

@@ -45,10 +45,16 @@ export function useTimelineStackingSync({ expandedElementsRef }: UseTimelineStac
     [zSyncPreviewIframeRef, zSyncActiveCompPath],
   );
 
+  // NaN (NOT 0) when the element can't be resolved in the preview iframe — a
+  // nested / unmounted sub-comp node, or one outside the active file. Fabricating
+  // z=0 would enter computeStackingPatches as a real overlapping neighbour at the
+  // z-floor and skew the boundary math; a non-finite value tells it to EXCLUDE this
+  // clip instead. NaN (rather than null) keeps the return assignable to the
+  // `(el) => number` reader contract the drag hook / commit deps declare.
   const readClipZIndex = useCallback(
     (el: TimelineElement): number => {
       const node = resolveIframeElement(el);
-      return node ? readEffectiveZIndex(node) : 0;
+      return node ? readEffectiveZIndex(node) : Number.NaN;
     },
     [resolveIframeElement],
   );
@@ -71,7 +77,9 @@ export function useTimelineStackingSync({ expandedElementsRef }: UseTimelineStac
           },
         ];
       });
-      if (entries.length) return handleDomZIndexReorderCommit(entries, coalesceKey);
+      // Forward the drag-commit's shared coalesce key so the z-reorder history
+      // entry merges with the lane change's move entry into one undo step.
+      if (entries.length) handleDomZIndexReorderCommit(entries, coalesceKey);
     },
     [handleDomZIndexReorderCommit, resolveIframeElement, zSyncActiveCompPath, expandedElementsRef],
   );