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.Application/Ports.cs

@@ -24,7 +24,10 @@ public interface IWorkflowClient
 /// </summary>
 public interface IAclClient
 {
-    Task<Uri> OpenZaakAsync(string bsn, CancellationToken ct = default);
+    /// <summary>Open a zaak for the registration. <paramref name="reference"/> is the registration's
+    /// own reference (its id); the ACL records it as the zaak's identificatie so the openbaar register
+    /// can show the same reference the zorgprofessional was given (S-09b follow-up, adr-proposal #78).</summary>
+    Task<Uri> OpenZaakAsync(string bsn, string reference, CancellationToken ct = default);
 
     /// <summary>
     /// Record the approval on the given zaak. The ACL translates this to the ZGW concept — setting