test(planning): compare the engine against a frozen engine on random requests

Bundle the planning core at a871f8f (develop after PR #278) as a frozen
fixture and compare it with the current engine on 5,000 seeded random
requests. Outcomes, stable explanations, explanation errors and final-state
snapshots must be strictly deep-equal. Requests cover obligations,
preferences, IF_THEN and TIME_WINDOW rules, earned entitlements, composed
optimization rules, declared days, year boundaries and public holidays.

Refs #280

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

src/app/services/frozen-engine/leave-planner.core.frozen.d.mts

@@ -0,0 +1,7 @@
+import type { PlanningArtifactsCallback, PlanningOutcome } from '../leave-planner.core';
+
+export function calculatePlanningInternal(
+	request: unknown,
+	checkpointCallback?: undefined,
+	artifactsCallback?: PlanningArtifactsCallback,
+): PlanningOutcome;