feat(domain): pass the registrationId to the ACL as the zaak reference (refs #78)

OpenZaakWorker forwards registration.Id to IAclClient.OpenZaakAsync so the zaak's
identificatie equals the reference the citizen sees on the submit confirmation.

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

tests/acceptance/Support/InMemoryDomainPorts.cs

@@ -26,11 +26,13 @@ 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 string? OpenedWithReference { get; private set; }
     public Uri? ApprovedZaakUrl { get; private set; }
 
-    public Task<Uri> OpenZaakAsync(string bsn, CancellationToken ct = default)
+    public Task<Uri> OpenZaakAsync(string bsn, string reference, CancellationToken ct = default)
     {
         OpenedForBsn = bsn;
+        OpenedWithReference = reference;
         return Task.FromResult(OpenedZaakUrl);
     }