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.Tests/Fakes.cs

@@ -0,0 +1,60 @@
+using Big.Application;
+using Big.Domain;
+
+namespace Big.Tests;
+
+/// <summary>An in-memory <see cref="IRegistrationStore"/> for the application-layer tests. Upserts
+/// keyed on the registration id, mirroring the production in-memory store.</summary>
+internal sealed class InMemoryRegistrationStore : IRegistrationStore
+{
+    private readonly Dictionary<RegistrationId, Registration> _byId = [];
+
+    public int SaveCount { get; private set; }
+
+    public Task SaveAsync(Registration registration, CancellationToken ct = default)
+    {
+        SaveCount++;
+        _byId[registration.Id] = registration;
+        return Task.CompletedTask;
+    }
+
+    public Task<Registration?> GetAsync(RegistrationId id, CancellationToken ct = default)
+        => Task.FromResult(_byId.GetValueOrDefault(id));
+
+    public void Seed(Registration registration) => _byId[registration.Id] = registration;
+}
+
+/// <summary>A fake Workflow Client that records the registration it was asked to start a process for
+/// and returns a fixed process-instance id. An optional callback runs at start time, letting a test
+/// assert ordering (e.g. that the registration was persisted before the process started).</summary>
+internal sealed class FakeWorkflowClient(string processInstanceId = "proc-1", Action<RegistrationId>? onStart = null)
+    : IWorkflowClient
+{
+    public RegistrationId? StartedFor { get; private set; }
+
+    public Task<string> StartRegistrationProcessAsync(RegistrationId registrationId, CancellationToken ct = default)
+    {
+        onStart?.Invoke(registrationId);
+        StartedFor = registrationId;
+        return Task.FromResult(processInstanceId);
+    }
+}
+
+/// <summary>A fake ACL client that records the bsn it was asked to open a zaak for and returns a
+/// fixed zaak URL.</summary>
+internal sealed class FakeAclClient(Uri? zaakUrl = null) : IAclClient
+{
+    public static readonly Uri DefaultZaakUrl = new("http://openzaak/zaken/api/v1/zaken/abc");
+
+    private readonly Uri _zaakUrl = zaakUrl ?? DefaultZaakUrl;
+
+    public string? OpenedForBsn { get; private set; }
+    public int CallCount { get; private set; }
+
+    public Task<Uri> OpenZaakAsync(string bsn, CancellationToken ct = default)
+    {
+        CallCount++;
+        OpenedForBsn = bsn;
+        return Task.FromResult(_zaakUrl);
+    }
+}