perf(planning): copy only the debited balance when assigning a date

Assigning a date copied every balance and every earned activation.
Balances and activations are only edited right after a state copies
them, so the new state now shares them with its parent, except the
balance it debits. Placement copying is unchanged.

Part of #302.

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

docs/benchmark/issue-302.md

@@ -108,3 +108,26 @@ the sum. The core therefore scores them without revalidating;
 | loaded year                 |      48.12 |     44.03 |
 | two years                   |       1.04 |      0.96 |
 | three years                 |       3.35 |      3.03 |
+
+### Copy only the debited balance when assigning a date
+
+Assigning a date copied every balance and every earned activation. Balances and
+activations are only edited right after a state copies them, so the new state
+now shares them with its parent, except the balance it debits. Placement copying
+is unchanged. The benchmark scenarios have at most six balances and few earned
+activations, so the gain is within run-to-run noise.
+
+| Scenario                    | Before (s) | After (s) |
+| --------------------------- | ---------: | --------: |
+| representative year         |       0.29 |      0.28 |
+| typed obligations           |       3.38 |      3.27 |
+| chained earned entitlements |       0.01 |      0.01 |
+| deliberate conflict         |       0.00 |      0.00 |
+| loaded year                 |      43.93 |     44.05 |
+| two years                   |       0.95 |      0.92 |
+| three years                 |       3.00 |      2.97 |
+
+Across the five exact steps, the same harness measured three years at
+7.45 seconds before the first step and 2.97 seconds after the last, two years at
+2.27 and 0.92 seconds, typed obligations at 5.11 and 3.27 seconds, and the loaded
+year at 58.26 and 44.05 seconds. These figures come from different sessions.

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

@@ -696,22 +696,24 @@ function reason(code: string, message: string, ruleIds?: string[]): PlanningReas
 	return ruleIds?.length ? { code, message, ruleIds } : { code, message };
 }
 
-function cloneState(state: State, placements?: State['placements']): State {
-	const clonedPlacements =
-		placements ??
-		new Map(
-			[...state.placements.entries()].map(([date, placement]) => [
-				date,
-				{
-					...placement,
-					ruleIds: new Set(placement.ruleIds),
-				},
-			]),
-		);
-	derivePlacements(state.placements, clonedPlacements);
+function copyPlacements(state: State): State['placements'] {
+	return new Map(
+		[...state.placements.entries()].map(([date, placement]) => [
+			date,
+			{
+				...placement,
+				ruleIds: new Set(placement.ruleIds),
+			},
+		]),
+	);
+}
+
+function cloneState(state: State): State {
+	const placements = copyPlacements(state);
+	derivePlacements(state.placements, placements);
 	return {
 		balances: state.balances.map((balance) => ({ ...balance })),
-		placements: clonedPlacements,
+		placements,
 		earnedActivations: state.earnedActivations.map((activation) => ({
 			...activation,
 			dates: [...activation.dates],
@@ -719,6 +721,16 @@ function cloneState(state: State, placements?: State['placements']): State {
 	};
 }
 
+/**
+ * Balances and activations are only edited right after a state copies them, so a
+ * new placement shares them with its parent state, except the balance it debits.
+ */
+function debitBalance(state: State, balanceId: string): InternalBalance[] {
+	return state.balances.map((balance) =>
+		balance.id === balanceId ? { ...balance, remaining: balance.remaining - 1 } : balance,
+	);
+}
+
 function planningStateSnapshot(state: State, context: SearchContext): PlanningStateSnapshot {
 	return {
 		calculationDate: context.request.calculationDate,
@@ -1458,18 +1470,20 @@ function assignDateVariants(
 			return compareDate(a.expiryDate, b.expiryDate) || compareText(a.id, b.id);
 		});
 	return balances.map((balance) => {
-		const result = cloneState(state);
-		const target = result.balances.find((item) => item.id === balance.id)!;
-		target.remaining--;
-		notePlacementAdded(state.placements, result.placements, date);
-		result.placements.set(date, {
+		const placements = copyPlacements(state);
+		notePlacementAdded(state.placements, placements, date);
+		placements.set(date, {
 			date,
-			balanceId: target.id,
-			type: target.type,
+			balanceId: balance.id,
+			type: balance.type,
 			ruleIds: ruleId ? new Set([ruleId]) : new Set(),
 			supporting: false,
 		});
-		return result;
+		return {
+			balances: debitBalance(state, balance.id),
+			placements,
+			earnedActivations: state.earnedActivations,
+		};
 	});
 }
 
@@ -1497,18 +1511,20 @@ function assignDateToBalance(
 	// 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)!;
-	target.remaining--;
-	notePlacementAdded(state.placements, result.placements, date);
-	result.placements.set(date, {
+	const placements = new Map(state.placements);
+	notePlacementAdded(state.placements, placements, date);
+	placements.set(date, {
 		date,
 		balanceId,
 		type: balance.type,
 		ruleIds: new Set(ruleIds),
 		supporting: false,
 	});
-	return result;
+	return {
+		balances: debitBalance(state, balanceId),
+		placements,
+		earnedActivations: state.earnedActivations,
+	};
 }
 
 function addEarnedBalance(