perf(planning): update plan observations incrementally

Cache an immutable occupancy per placements map and derive a candidate's
time-off period groups, holiday-week completeness and placement tie key
from its parent when one date is added or removed. The from-scratch
computation stays as the test oracle and an opt-in shadow mode. A
fast-check property test compares both after every operation.

The loaded year drops from 66.5 s to 48.5 s and the protocol from 262.0 s
to 193.7 s. Outcomes, explanations and final states stay identical.

Refs #283

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

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

@@ -13,6 +13,9 @@ import {
 	optimizationBalanceObservations,
 	optimizationObservations,
 	periodData,
+	derivePlacements,
+	notePlacementAdded,
+	notePlacementRemoved,
 	placementTieKey,
 	utcDate,
 	workSequenceDates,
@@ -698,19 +701,21 @@ function reason(code: string, message: string, ruleIds?: string[]): PlanningReas
 }
 
 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);
 	return {
 		balances: state.balances.map((balance) => ({ ...balance })),
-		placements:
-			placements ??
-			new Map(
-				[...state.placements.entries()].map(([date, placement]) => [
-					date,
-					{
-						...placement,
-						ruleIds: new Set(placement.ruleIds),
-					},
-				]),
-			),
+		placements: clonedPlacements,
 		earnedActivations: state.earnedActivations.map((activation) => ({
 			...activation,
 			dates: [...activation.dates],
@@ -1460,6 +1465,7 @@ function assignDateVariants(
 		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, {
 			date,
 			balanceId: target.id,
@@ -1498,6 +1504,7 @@ function assignDateToBalance(
 	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, {
 		date,
 		balanceId,
@@ -1838,6 +1845,7 @@ function releasePlacement(state: State, date: string): State {
 		})),
 	};
 	const placement = result.placements.get(date)!;
+	notePlacementRemoved(state.placements, result.placements, date);
 	result.placements.delete(date);
 	result.balances.find((balance) => balance.id === placement.balanceId)!.remaining++;
 	return result;
@@ -2811,7 +2819,7 @@ function* optimizeRulePlacements(
 						moved &&
 						dateInvariantObjectives &&
 						compareObjectiveLists(
-							tieBreakObjectives(moved),
+							tieBreakObjectives(moved, context),
 							beforeVector.objectives.slice(-2),
 						) >= 0
 					) {
@@ -3346,7 +3354,7 @@ function* activeObjectives(
 	}
 }
 
-function tieBreakObjectives(state: State): PlanningObjective[] {
+function tieBreakObjectives(state: State, context: SearchContext): PlanningObjective[] {
 	return [
 		{
 			id: 'balance-expiry',
@@ -3357,7 +3365,7 @@ function tieBreakObjectives(state: State): PlanningObjective[] {
 		{
 			id: 'placement-date',
 			direction: 'minimize',
-			value: placementTieKey(state),
+			value: placementTieKey(state, context),
 			label: 'Placement date tie-break',
 		},
 	];
@@ -3367,7 +3375,10 @@ function obligationPreviewObjectives(
 	preview: { state: State },
 	context: SearchContext,
 ): PlanningObjective[] {
-	return [...activeObjectiveVector(preview.state, context), ...tieBreakObjectives(preview.state)];
+	return [
+		...activeObjectiveVector(preview.state, context),
+		...tieBreakObjectives(preview.state, context),
+	];
 }
 
 /** Compare complete previews so obligation DFS does not commit to its first feasible branch. */
@@ -3553,7 +3564,7 @@ function objectiveVector(
 				label: 'Unsatisfied obligations',
 			},
 			...active,
-			...tieBreakObjectives(state),
+			...tieBreakObjectives(state, context),
 		],
 	};
 }
@@ -3692,6 +3703,7 @@ function planExplanation(
 					compareDate(normalizedBalance.expiryDate, context.horizon.end) <= 0
 				: false;
 			const withoutPlacement = cloneState(state);
+			notePlacementRemoved(state.placements, withoutPlacement.placements, placement.date);
 			withoutPlacement.placements.delete(placement.date);
 			const released = withoutPlacement.balances.find(
 				(candidate) => candidate.id === placement.balanceId,
@@ -4975,7 +4987,10 @@ export function analyzeCounterfactualInternal(
 	const baseline = cloneState(finalState);
 	const removeDate = selected.date;
 	const removed = removeDate ? baseline.placements.get(removeDate) : undefined;
-	if (removeDate) baseline.placements.delete(removeDate);
+	if (removeDate) {
+		notePlacementRemoved(finalState.placements, baseline.placements, removeDate);
+		baseline.placements.delete(removeDate);
+	}
 	const released = baseline.balances.find((balance) => balance.id === selected.balanceId);
 	if (released && removed?.balanceId === selected.balanceId) released.remaining++;
 	const beforeState = baseline;