feat(domain): withdrawal cancels the registratie process via a BPMN message event (refs #12)

Add an interrupting message boundary event (RegistratieIngetrokken) on the Beoordelen task that

ends the process; the Workflow Client delivers the message to the task's execution, and the

withdraw handler triggers it best-effort after the domain transition (ADR-0014).

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

services/domain/Big.Application/Ports.cs

@@ -54,11 +54,18 @@ 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) and the registration it carries as a process variable.</summary>
-public sealed record BeoordelingTask(string TaskId, RegistrationId RegistrationId);
+/// 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);
 
 /// <summary>
 /// Persistence port for the <see cref="Registration"/> aggregate. In-memory for the minimal slice