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.Api/Program.cs

@@ -48,7 +48,12 @@ app.MapGet("/health", () => "Healthy");
 // a location to read the registration's progress (ADR-0009, eventual consistency).
 app.MapPost("/registrations", async (SubmitRegistrationRequest body, SubmitRegistration submit, CancellationToken ct) =>
 {
-    var id = await submit.HandleAsync(new SubmitRegistrationCommand(body.Bsn), ct);
+    // Diploma origin defaults to domestic; a foreign (eIDAS) submission passes "Buitenlands" so the
+    // workflow's DMN routes it through CBGV-advies (S-13). An unknown value is a bad request.
+    if (!Enum.TryParse<DiplomaOrigin>(body.DiplomaOrigin, ignoreCase: true, out var origin) && body.DiplomaOrigin is not null)
+        return Results.BadRequest(new { error = $"Unknown diplomaOrigin '{body.DiplomaOrigin}'. Expected 'Binnenlands' or 'Buitenlands'." });
+
+    var id = await submit.HandleAsync(new SubmitRegistrationCommand(body.Bsn, origin), ct);
     return Results.Accepted($"/registrations/{id}", new RegistrationResponse(id.ToString(), RegistrationStatus.Ingediend.ToString(), null));
 });
 
@@ -117,7 +122,7 @@ app.MapGet("/registrations/{id}", async (string id, IRegistrationStore store, Ca
 
 await app.RunAsync();
 
-public sealed record SubmitRegistrationRequest(string Bsn);
+public sealed record SubmitRegistrationRequest(string Bsn, string? DiplomaOrigin = null);
 
 public sealed record DecideRequest(string Besluit);
 

services/domain/Big.Infrastructure/FlowableWorkflowClient.cs

@@ -24,6 +24,7 @@ public sealed class FlowableWorkflowClient(HttpClient http, FlowableOptions opti
     private const string BehandelaarGroup = "behandelaar";
     private const string TeamleadGroup = "teamlead";
     private const string RegistrationIdVariable = "registrationId";
+    private const string DiplomaOriginVariable = "diplomaOrigin";
     private const string ZaakUrlVariable = "zaakUrl";
     private const string BesluitVariable = "besluit";
     private const string IngetrokkenMessage = "RegistratieIngetrokken";
@@ -33,7 +34,10 @@ public sealed class FlowableWorkflowClient(HttpClient http, FlowableOptions opti
     {
         var request = new StartProcessRequest(
             ProcessDefinitionKey,
-            [new Variable(RegistrationIdVariable, "string", registrationId.ToString())]);
+            [
+                new Variable(RegistrationIdVariable, "string", registrationId.ToString()),
+                new Variable(DiplomaOriginVariable, "string", diplomaOrigin.ToString()),
+            ]);
 
         var created = await PostAsync<StartProcessRequest, ProcessInstance>(
             "service/runtime/process-instances", request, ct)

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()
     {