fix: use anyChanged flag to preserve array referential identity in useStableRows

The anyChanged variable was assigned but never read, causing a new array
reference to always be returned from useMemo even when all rows were
structurally identical. This partially defeated the purpose of the
structural sharing hook for LegendList's data prop.

Now we track the previous result and return it when no rows changed,
preserving referential identity and avoiding unnecessary list
reconciliation.

Author: cursoragent

apps/web/src/components/chat/MessagesTimeline.tsx

@@ -785,11 +785,12 @@ const UserMessageBody = memo(function UserMessageBody(props: {
  *  hasn't changed since last call, the previous object reference is reused. */
 function useStableRows(rows: MessagesTimelineRow[]): MessagesTimelineRow[] {
   const prevById = useRef(new Map<string, MessagesTimelineRow>());
+  const prevResult = useRef<MessagesTimelineRow[]>([]);
 
   return useMemo(() => {
     const prev = prevById.current;
     const next = new Map<string, MessagesTimelineRow>();
-    let anyChanged = false;
+    let anyChanged = rows.length !== prev.size;
 
     const result = rows.map((row) => {
       const prevRow = prev.get(row.id);
@@ -803,6 +804,9 @@ function useStableRows(rows: MessagesTimelineRow[]): MessagesTimelineRow[] {
     });
 
     prevById.current = next;
+
+    if (!anyChanged) return prevResult.current;
+    prevResult.current = result;
     return result;
   }, [rows]);
 }