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.Application/Ports.cs

@@ -11,10 +11,12 @@ public interface IWorkflowClient
 {
     /// <summary>
     /// Start one <c>registratie</c> process instance for the given registration, carrying the
-    /// registration id so the <c>OpenZaakAanmaken</c> external task can be correlated back to its
-    /// aggregate. Returns the process instance id.
+    /// registration id (so the <c>OpenZaakAanmaken</c> external task can be correlated back to its
+    /// aggregate) and the diploma origin (so the workflow's DMN can route foreign diplomas through
+    /// CBGV-advies, S-13). Returns the process instance id.
     /// </summary>
-    Task<string> StartRegistrationProcessAsync(RegistrationId registrationId, CancellationToken ct = default);
+    Task<string> StartRegistrationProcessAsync(
+        RegistrationId registrationId, DiplomaOrigin diplomaOrigin, CancellationToken ct = default);
 
     /// <summary>
     /// Cancel a running <c>registratie</c> process on withdrawal (S-11): correlate the