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/Ports.cs

@@ -0,0 +1,47 @@
+using Big.Domain;
+
+namespace Big.Application;
+
+/// <summary>
+/// The port to the workflow engine. Implemented by the Workflow Client in Infrastructure — the
+/// <em>only</em> code that talks to Flowable (CLAUDE.md §8.2). The application asks it to start the
+/// registratie process; it never knows Flowable exists.
+/// </summary>
+public interface IWorkflowClient
+{
+    /// <summary>
+    /// Start one <c>registratie</c> process instance for the given registration, carrying the
+    /// registration id so the <c>OpenZaakAanmaken</c> external task can be correlated back to its
+    /// aggregate. Returns the process instance id.
+    /// </summary>
+    Task<string> StartRegistrationProcessAsync(RegistrationId registrationId, CancellationToken ct = default);
+}
+
+/// <summary>
+/// The port to the Anti-Corruption Layer. Implemented in Infrastructure by an HTTP client to the
+/// ACL service — the <em>only</em> code that talks to ZGW (CLAUDE.md §8.1). The domain hands over a
+/// bsn; the ACL default-fills the ZGW-mandatory fields (ADR-0003) and returns the created zaak URL.
+/// </summary>
+public interface IAclClient
+{
+    Task<Uri> OpenZaakAsync(string bsn, CancellationToken ct = default);
+}
+
+/// <summary>
+/// Persistence port for the <see cref="Registration"/> aggregate. In-memory for the minimal slice
+/// (ADR-0009); an EF-backed store is a documented follow-up, and this port keeps that change additive.
+/// </summary>
+public interface IRegistrationStore
+{
+    /// <summary>Insert or update the registration, keyed on its id.</summary>
+    Task SaveAsync(Registration registration, CancellationToken ct = default);
+
+    /// <summary>Load a registration by id, or <c>null</c> if none exists.</summary>
+    Task<Registration?> GetAsync(RegistrationId id, CancellationToken ct = default);
+}
+
+/// <summary>
+/// An acquired <c>OpenZaakAanmaken</c> external-worker job: the Flowable job id (needed to complete
+/// it) and the registration id it carries as a process variable.
+/// </summary>
+public sealed record OpenZaakJob(string JobId, RegistrationId RegistrationId);