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/OpenZaakWorker.cs

@@ -0,0 +1,27 @@
+using Big.Domain;
+
+namespace Big.Application;
+
+/// <summary>
+/// Handles one acquired <c>OpenZaakAanmaken</c> external-worker job (ADR-0009): load the registration
+/// the job correlates to, open a zaak for it via the ACL (§8.1), attach the zaak to the aggregate, and
+/// return the zaak URL so the caller can complete the Flowable job. Pure application logic over ports —
+/// it knows nothing of Flowable; the polling loop that feeds it jobs lives in Infrastructure.
+/// </summary>
+public sealed class OpenZaakWorker(IRegistrationStore store, IAclClient acl)
+{
+    /// <summary>
+    /// Process the job and return the URL of the (existing or newly opened) zaak. Idempotent: if the
+    /// registration already has a zaak — a job redelivered after its completion was lost — it returns
+    /// that zaak without opening a second one (§8.6, at-least-once delivery). An unknown registration
+    /// is an error: it throws, leaving the job un-completed for Flowable to redeliver.
+    /// </summary>
+    public async Task<Uri> HandleAsync(OpenZaakJob job, CancellationToken ct = default)
+    {
+        ArgumentNullException.ThrowIfNull(job);
+
+        // STUB (red): does not load, call the ACL, or attach.
+        await Task.CompletedTask;
+        return new Uri("about:blank");
+    }
+}