test(workflow): start carries diploma origin as a process variable (S-13, refs #14)

Add DiplomaOrigin (Binnenlands/Buitenlands) to the Registration aggregate and
submit command, and thread it through the process-start port so the workflow's
DMN can route on it (ADR proposal #100). Failing Workflow Client test asserts the
diplomaOrigin start variable; the client takes the origin but does not emit it yet.

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

services/domain/Big.Tests/Fakes.cs

@@ -32,12 +32,15 @@ internal sealed class FakeWorkflowClient(string processInstanceId = "proc-1", Ac
     : IWorkflowClient
 {
     public RegistrationId? StartedFor { get; private set; }
+    public DiplomaOrigin? StartedWithOrigin { get; private set; }
     public string? WithdrawnProcessInstanceId { get; private set; }
 
-    public Task<string> StartRegistrationProcessAsync(RegistrationId registrationId, CancellationToken ct = default)
+    public Task<string> StartRegistrationProcessAsync(
+        RegistrationId registrationId, DiplomaOrigin diplomaOrigin, CancellationToken ct = default)
     {
         onStart?.Invoke(registrationId);
         StartedFor = registrationId;
+        StartedWithOrigin = diplomaOrigin;
         return Task.FromResult(processInstanceId);
     }