perf(planning): reuse the donor ranking across public-holiday weeks

The holiday-week stage ranked every placement as a donor once per
public-holiday week, although the ranking depends only on the plan. The
weeks of one plan now share each placement's released objectives; each
week still skips its own dates and walks the placements in the same
order with the same yields.

Part of #302.

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

docs/benchmark/issue-302.md

@@ -0,0 +1,37 @@
+# Issue #302: plan from the best-ranked obligation solution
+
+Step-by-step record for [issue #302](https://gitea.nessar.fr/nessar/vac-optimizer/issues/302).
+The exact steps keep planning outcomes, stable explanations and final states
+identical to the frozen engine at `a871f8f`. The
+[frozen-engine differential check](README.md#frozen-engine-differential-check),
+including its cancelled asynchronous comparisons, passes after every step.
+
+## Per-step measurements
+
+Each step was timed against its parent in the same session, on the development
+machine (AMD Ryzen 7 7800X3D, WSL2, Bun 1.4.3), with no other test, build or
+profiling workload running. Both commits' cores were bundled with the
+frozen-engine build command and called through `calculatePlanningAsyncInternal`
+in one Bun process, with `calculationDate: '2024-01-01'`. This is the
+benchmark's in-thread asynchronous path, without the planning-calculation
+service. For every scenario, each engine ran one warm-up and then one measured
+calculation, alternating engines. Outcomes, explanations and final states were
+hashed; every pair below matched. Only same-session pairs are comparable.
+
+### Reuse the donor ranking across public-holiday weeks
+
+For every public-holiday week, the holiday-week stage ranked every placement as
+a potential donor by releasing it and evaluating the active objectives. That
+ranking depends only on the plan, so the weeks of one plan now share each
+placement's released objectives. Each week still skips its own dates and walks
+the placements in the same order with the same yields.
+
+| Scenario                    | Before (s) | After (s) |
+| --------------------------- | ---------: | --------: |
+| representative year         |       0.66 |      0.66 |
+| typed obligations           |       5.11 |      4.89 |
+| chained earned entitlements |       0.02 |      0.02 |
+| deliberate conflict         |       0.01 |      0.01 |
+| loaded year                 |      58.26 |     57.75 |
+| two years                   |       2.27 |      1.82 |
+| three years                 |       7.45 |      5.52 |

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

@@ -2208,6 +2208,7 @@ function* calendarPlacementCandidates(
 	ruleId: string,
 	context: SearchContext,
 	accept: (candidate: State, addition?: boolean) => boolean,
+	donorObjectives = new Map<string, PlanningObjective[]>(),
 ): PlanningSteps<boolean> {
 	const missing = targetDates.filter(
 		(date) => !state.placements.has(date) && !context.declaredDates.has(date),
@@ -2221,10 +2222,12 @@ function* calendarPlacementCandidates(
 		yield;
 		context.check();
 		if (targetDates.includes(placement.date)) continue;
-		rankedDonors.push({
-			date: placement.date,
-			objectives: activeObjectiveVector(releasePlacement(state, placement.date), context),
-		});
+		let objectives = donorObjectives.get(placement.date);
+		if (!objectives) {
+			objectives = activeObjectiveVector(releasePlacement(state, placement.date), context);
+			donorObjectives.set(placement.date, objectives);
+		}
+		rankedDonors.push({ date: placement.date, objectives });
 	}
 	// shortcut: recursion retains the original donor ranking; rerank when a measured bundle misses a better allocation.
 	rankedDonors.sort(
@@ -2330,8 +2333,17 @@ function* holidayWeekPlacementCandidates(
 	context: SearchContext,
 	accept: (candidate: State, addition?: boolean) => boolean,
 ): PlanningSteps<void> {
+	// Releasing a donor scores the same for every week of this plan.
+	const donorObjectives = new Map<string, PlanningObjective[]>();
 	for (const week of context.holidayWeeks)
-		yield* calendarPlacementCandidates(state, week.dates, ruleId, context, accept);
+		yield* calendarPlacementCandidates(
+			state,
+			week.dates,
+			ruleId,
+			context,
+			accept,
+			donorObjectives,
+		);
 }
 
 function* timeOffPeriodPlacementCandidates(