fix(rack): memoize dnd-kit sensors and prevent pointermove state thrashing

client/src/components/rack/RackPlanner.tsx

@@ -79,6 +79,10 @@ function resolveSlotFromPoint(clientX: number, clientY: number): HoverSlot | nul
   return { rackId, uPosition: uPos };
 }
 
+const POINTER_SENSOR_OPTIONS = {
+  activationConstraint: { distance: 6 },
+};
+
 export function RackPlanner() {
   const { racks, loading, fetchRacks, moveModule } = useRackStore();
   const canvasRef = useRef<HTMLDivElement>(null);
@@ -105,7 +109,7 @@ export function RackPlanner() {
   }
 
   const sensors = useSensors(
-    useSensor(PointerSensor, { activationConstraint: { distance: 6 } })
+    useSensor(PointerSensor, POINTER_SENSOR_OPTIONS)
   );
 
   useEffect(() => {
@@ -122,9 +126,13 @@ export function RackPlanner() {
     function onPointerMove(e: PointerEvent) {
       if (!isDraggingAnyRef.current) return;
       const slot = resolveSlotFromPoint(e.clientX, e.clientY);
+      
+      const prev = hoverSlotRef.current;
+      if (prev?.rackId !== slot?.rackId || prev?.uPosition !== slot?.uPosition) {
         hoverSlotRef.current = slot;
         setHoverSlot(slot);
       }
+    }
 
     // Capture phase so we get the event before any element can stop propagation.
     window.addEventListener('pointermove', onPointerMove, { capture: true });