fix(domain): correlate withdrawal by process instance to the subscribed execution (refs #12)

verify-stack surfaced a Flowable 500: delivering messageEventReceived to the Beoordelen task's

execution is wrong — a message boundary event's subscription lives on its own execution. Correlate

instead by the registration's process instance id (recorded at submit): query the execution

subscribed to RegistratieIngetrokken and deliver the message there. Withdrawal moves from

IUserTaskClient to IWorkflowClient.WithdrawProcessAsync; the handler no longer needs a task lookup.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

services/domain/Big.Application/Ports.cs

@@ -15,6 +15,14 @@ public interface IWorkflowClient
     /// aggregate. Returns the process instance id.
     /// </summary>
     Task<string> StartRegistrationProcessAsync(RegistrationId registrationId, CancellationToken ct = default);
+
+    /// <summary>
+    /// Cancel a running <c>registratie</c> process on withdrawal (S-11): correlate the
+    /// <c>RegistratieIngetrokken</c> message to the instance, tripping the interrupting message event
+    /// that ends it (ADR-0014). Best-effort — if the instance is not waiting on that message (already
+    /// ended, or not yet parked) it is a no-op; the aggregate is INGETROKKEN regardless.
+    /// </summary>
+    Task WithdrawProcessAsync(string processInstanceId, CancellationToken ct = default);
 }
 
 /// <summary>
@@ -54,18 +62,11 @@ public interface IUserTaskClient
     /// <summary>Complete a beoordeling task, carrying the decision into the process as the
     /// <c>besluit</c> variable so the workflow can continue on the chosen branch.</summary>
     Task CompleteBeoordelingAsync(string taskId, BeoordelingsBesluit besluit, CancellationToken ct = default);
-
-    /// <summary>Deliver the withdrawal message to a <c>Beoordelen</c> task's execution, tripping the
-    /// process's interrupting message boundary event so the registratie process cancels (S-11,
-    /// ADR-0014). The registration itself is already INGETROKKEN in the domain; this ends its
-    /// workflow so the case leaves the werkbak.</summary>
-    Task WithdrawBeoordelingAsync(string executionId, CancellationToken ct = default);
 }
 
 /// <summary>A <c>Beoordelen</c> user task in the werkbak: the Flowable task id (needed to claim and
-/// complete it), the execution it runs in (needed to deliver the withdrawal message to its boundary
-/// event), and the registration it carries as a process variable.</summary>
-public sealed record BeoordelingTask(string TaskId, string ExecutionId, RegistrationId RegistrationId);
+/// complete it) and the registration it carries as a process variable.</summary>
+public sealed record BeoordelingTask(string TaskId, RegistrationId RegistrationId);
 
 /// <summary>
 /// Persistence port for the <see cref="Registration"/> aggregate. In-memory for the minimal slice