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);