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.Infrastructure/FlowableWorkflowClient.cs

@@ -28,7 +28,8 @@ public sealed class FlowableWorkflowClient(HttpClient http, FlowableOptions opti
     private const string BesluitVariable = "besluit";
     private const string IngetrokkenMessage = "RegistratieIngetrokken";
 
-    public async Task<string> StartRegistrationProcessAsync(RegistrationId registrationId, CancellationToken ct = default)
+    public async Task<string> StartRegistrationProcessAsync(
+        RegistrationId registrationId, DiplomaOrigin diplomaOrigin, CancellationToken ct = default)
     {
         var request = new StartProcessRequest(
             ProcessDefinitionKey,