test(domain): acceptance scenario for submitting a registration (refs #6)

Use-case-level BDD (Reqnroll) for S-05: a zorgprofessional submits a registration;
the Domain Service starts the registratie process and the OpenZaakAanmaken external
task opens a zaak via the ACL, recorded on the aggregate (ADR-0009). Driven against
in-memory Workflow Client and ACL stand-ins; real Flowable+ACL+OpenZaak delivery is
the live-stack verify-domain check.

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

tests/acceptance/Support/InMemoryDomainPorts.cs

@@ -0,0 +1,50 @@
+using Big.Application;
+using Big.Domain;
+
+namespace Acceptance.Support;
+
+/// <summary>An in-memory Workflow Client for the domain acceptance scenario: it records which
+/// registration a process was started for and returns a fixed instance id. The acceptance test
+/// drives the use case without a running Flowable (live verification is the verify-domain check).</summary>
+public sealed class InMemoryWorkflowClient : IWorkflowClient
+{
+    public const string StartedProcessInstanceId = "proc-acc-1";
+
+    public RegistrationId? StartedFor { get; private set; }
+
+    public Task<string> StartRegistrationProcessAsync(RegistrationId registrationId, CancellationToken ct = default)
+    {
+        StartedFor = registrationId;
+        return Task.FromResult(StartedProcessInstanceId);
+    }
+}
+
+/// <summary>An in-memory ACL stand-in: records the bsn it opened a zaak for and returns a fixed URL,
+/// so the scenario verifies the domain crosses the ACL boundary (§8.1) without a running OpenZaak.</summary>
+public sealed class InMemoryAclClient : IAclClient
+{
+    public static readonly Uri OpenedZaakUrl = new("http://openzaak/zaken/api/v1/zaken/acc-zaak");
+
+    public string? OpenedForBsn { get; private set; }
+
+    public Task<Uri> OpenZaakAsync(string bsn, CancellationToken ct = default)
+    {
+        OpenedForBsn = bsn;
+        return Task.FromResult(OpenedZaakUrl);
+    }
+}
+
+/// <summary>An in-memory registration store for the domain acceptance scenario.</summary>
+public sealed class InMemoryRegistrationStore : IRegistrationStore
+{
+    private readonly Dictionary<RegistrationId, Registration> _byId = [];
+
+    public Task SaveAsync(Registration registration, CancellationToken ct = default)
+    {
+        _byId[registration.Id] = registration;
+        return Task.CompletedTask;
+    }
+
+    public Task<Registration?> GetAsync(RegistrationId id, CancellationToken ct = default)
+        => Task.FromResult(_byId.GetValueOrDefault(id));
+}