refactor(planning): give planning workers one lifetime owner

Planning, counterfactual analysis and checkpoint explanation now share
one internal worker-job owner for creation, handler cleanup,
termination and supersession, and one request/response message
contract between the coordinator and the worker. Job-specific progress,
checkpoint recovery and completion rules stay explicit.

Refs #277

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

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

@@ -1,15 +1,7 @@
 /// <reference lib="webworker" />
 
-import type {
-	CounterfactualAnalysis,
-	LeavePlacement,
-	PlanOutcome,
-	PlanningExplanation,
-	PlanningOutcome,
-	PlanningReason,
-	PlanningSnapshot,
-	PlanningStateSnapshot,
-} from './leave-planner.service';
+import type { PlanningOutcome } from './leave-planner.service';
+import type { WorkerRequest, WorkerResponse } from './leave-planner.worker-messages';
 import {
 	analyzeCounterfactualInternal,
 	calculatePlanningAsyncInternal,
@@ -17,54 +9,6 @@ import {
 	type PlanningArtifacts,
 } from './leave-planner.core';
 
-type WorkerPlanningRequest = {
-	calculationDate?: string;
-	declaredDays?: unknown;
-	horizon?: { start?: unknown; end?: unknown };
-	balances?: unknown;
-	rules?: unknown;
-	identities?: unknown;
-	labels?: unknown;
-	deadlineMs?: unknown;
-	progress?: unknown;
-};
-
-type WorkerRequest =
-	| {
-			type: 'plan';
-			request: WorkerPlanningRequest;
-	  }
-	| { type: 'cancel' }
-	| {
-			type: 'counterfactual';
-			request: PlanningSnapshot;
-			plan: PlanOutcome;
-			placement: LeavePlacement;
-			state?: PlanningStateSnapshot;
-	  }
-	| {
-			type: 'explain-checkpoint';
-			request: PlanningSnapshot;
-			plan: PlanOutcome;
-			state: PlanningStateSnapshot;
-			interruption: PlanningReason;
-	  };
-
-type WorkerResponse =
-	| { type: 'heartbeat' }
-	| { type: 'progress'; stage: string }
-	| { type: 'checkpoint'; plan: PlanOutcome; state?: PlanningStateSnapshot }
-	| { type: 'result'; outcome: PlanningOutcome }
-	| { type: 'artifacts'; artifacts: PlanningArtifacts }
-	| {
-			type: 'explanation';
-			explanation?: PlanningExplanation;
-			explanationError?: PlanningReason;
-			finalState?: PlanningStateSnapshot;
-	  }
-	| { type: 'counterfactual-result'; analysis: CounterfactualAnalysis }
-	| { type: 'error'; code?: string; message: string };
-
 const worker = self as DedicatedWorkerGlobalScope;
 let activeCalculation: { cancelled: boolean } | undefined;