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>

services/domain/Big.Tests/OpenZaakWorkerTests.cs

@@ -25,6 +25,9 @@ public class OpenZaakWorkerTests
 
         Assert.Equal(FakeAclClient.DefaultZaakUrl, zaakUrl);
         Assert.Equal("123456782", acl.OpenedForBsn);
+        // The registration's own id is handed to the ACL as the zaak reference (identificatie),
+        // so the openbaar register can show the reference the zorgprofessional received (#78).
+        Assert.Equal(registration.Id.ToString(), acl.OpenedWithReference);
         var saved = await store.GetAsync(registration.Id);
         Assert.Equal(FakeAclClient.DefaultZaakUrl, saved!.ZaakUrl);
         Assert.Equal(1, store.SaveCount);