fix(studio): propagate z-index reorder save failures and drop dead targetTrack

handleDomZIndexReorderCommit no longer swallows per-entry save failures: it settles every
patch, and on any rejection rolls back the eager DOM z-index/position and the optimistic
store zIndex before rejecting, so a failed save cannot leave the UI showing a stacking order
that never persisted or let an ordered-after timing write proceed.

Also removes the dead targetTrack parameter threaded through the timeline edit helpers;
vertical placement is owned by the z-index intent.

packages/studio/src/hooks/timelineEditingHelpers.test.ts

@@ -39,7 +39,6 @@ describe("applyTimelineStackingReorder", () => {
 
     applyTimelineStackingReorder({
       element: el({ id: "chip", tag: "div" }),
-      targetTrack: 0,
       stackingReorder: {
         contextKey: "scene",
         placement: { type: "above", layerId: "layer:scene:x" },
@@ -76,7 +75,6 @@ describe("applyTimelineStackingReorder", () => {
 
     applyTimelineStackingReorder({
       element: el({ id: "track", tag: "audio" }),
-      targetTrack: 0,
       stackingReorder: {
         contextKey: "main",
         placement: { type: "above", layerId: "layer:main:x" },