perf(planning): reject impossible mandatory moves before copying

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

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

@@ -1514,6 +1514,8 @@ function assignDateToBalance(
 	) {
 		return null;
 	}
+	// Reject impossible mandatory moves before copying; validate complete patterns later.
+	if (!mandatoryRelocationDate(date, balance, ruleIds, context)) return null;
 	// Existing placements stay read-only; this assignment owns only its new placement.
 	const result = cloneState(state, new Map(state.placements));
 	const target = result.balances.find((item) => item.id === balanceId)!;