test(domain): SubmitRegistration + OpenZaakWorker use cases (refs #6)

Failing application-layer tests over fake ports (IWorkflowClient, IAclClient,
IRegistrationStore):
- Submit persists an INGEDIEND registration and starts the registratie process,
  recording the instance id — and persists *before* starting, so the worker can
  correlate the OpenZaakAanmaken job back to its aggregate (ADR-0009).
- The worker opens a zaak via the ACL and attaches it; an unknown registration
  throws (job left for redelivery); a redelivered job is idempotent and opens no
  second zaak (§8.6).

Handlers are stubs (no persistence / no ACL call) so the tests compile and fail
on their assertions; the green commit implements them.

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

services/domain/Big.Application/SubmitRegistration.cs

@@ -0,0 +1,25 @@
+using Big.Domain;
+
+namespace Big.Application;
+
+/// <summary>A zorgprofessional's request to register, in domain language. No ZGW concepts.</summary>
+public sealed record SubmitRegistrationCommand(string Bsn);
+
+/// <summary>
+/// The submit use case: create the <see cref="Registration"/> aggregate (INGEDIEND), persist it,
+/// then start the registratie workflow process and record its instance id. It returns as soon as
+/// the process is started — opening the zaak happens later, off the request path, in the external-task
+/// worker (ADR-0009). Persisting <em>before</em> starting the process closes the race where the worker
+/// acquires the OpenZaakAanmaken job before the aggregate it correlates to exists.
+/// </summary>
+public sealed class SubmitRegistration(IRegistrationStore store, IWorkflowClient workflow)
+{
+    public async Task<RegistrationId> HandleAsync(SubmitRegistrationCommand command, CancellationToken ct = default)
+    {
+        ArgumentNullException.ThrowIfNull(command);
+
+        // STUB (red): no persistence, no process start.
+        await Task.CompletedTask;
+        return RegistrationId.New();
+    }
+}