perf(planning): build placement date keys directly

src/app/services/leave-planner.core.ts

@@ -3317,10 +3317,7 @@ function preferenceStateKey(state: State): string {
 }
 
 function placementTieKey(state: State): string {
-	return [...state.placements.values()]
-		.map((placement) => placement.date)
-		.sort(compareDate)
-		.join('|');
+	return [...state.placements.keys()].sort().join('|');
 }
 
 function expiryTieKey(state: State): string {