feat(workflow): emit diplomaOrigin start variable + accept it on submit (S-13, refs #14)

The Workflow Client now posts diplomaOrigin alongside registrationId when starting
the process, and the domain submit endpoint accepts an optional diplomaOrigin
(defaulting to domestic) so a foreign submission can be driven end-to-end. Adds the
aggregate + submit-forwarding tests now that the plumbing is in place.

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

services/domain/Big.Tests/RegistrationTests.cs

@@ -16,6 +16,15 @@ public class RegistrationTests
         Assert.Null(registration.ProcessInstanceId);
     }
 
+    [Fact]
+    public void A_registration_defaults_to_a_domestic_diploma()
+        => Assert.Equal(DiplomaOrigin.Binnenlands, Registration.Submit("123456782").DiplomaOrigin);
+
+    [Fact]
+    public void A_foreign_diploma_submission_records_its_origin()
+        => Assert.Equal(DiplomaOrigin.Buitenlands,
+            Registration.Submit("123456782", DiplomaOrigin.Buitenlands).DiplomaOrigin);
+
     [Theory]
     [InlineData("")]
     [InlineData("   ")]

services/domain/Big.Tests/SubmitRegistrationTests.cs

@@ -25,6 +25,19 @@ public class SubmitRegistrationTests
         Assert.Equal(2, store.SaveCount);
     }
 
+    [Fact]
+    public async Task Submitting_a_foreign_diploma_carries_its_origin_to_the_process()
+    {
+        var store = new FakeRegistrationStore();
+        var workflow = new FakeWorkflowClient();
+        var handler = new SubmitRegistration(store, workflow);
+
+        var id = await handler.HandleAsync(new SubmitRegistrationCommand("123456782", DiplomaOrigin.Buitenlands));
+
+        Assert.Equal(DiplomaOrigin.Buitenlands, (await store.GetAsync(id))!.DiplomaOrigin);
+        Assert.Equal(DiplomaOrigin.Buitenlands, workflow.StartedWithOrigin);
+    }
+
     [Fact]
     public async Task Rejects_a_null_command_without_touching_the_store_or_workflow()
     {