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/Fakes.cs

@@ -50,15 +50,17 @@ internal sealed class FakeAclClient(Uri? zaakUrl = null) : IAclClient
     private readonly Uri _zaakUrl = zaakUrl ?? DefaultZaakUrl;
 
     public string? OpenedForBsn { get; private set; }
+    public string? OpenedWithReference { get; private set; }
     public int CallCount { get; private set; }
 
     public Uri? ApprovedZaakUrl { get; private set; }
     public int ApproveCallCount { get; private set; }
 
-    public Task<Uri> OpenZaakAsync(string bsn, CancellationToken ct = default)
+    public Task<Uri> OpenZaakAsync(string bsn, string reference, CancellationToken ct = default)
     {
         CallCount++;
         OpenedForBsn = bsn;
+        OpenedWithReference = reference;
         return Task.FromResult(_zaakUrl);
     }