feat(acl): set and read the zaak identificatie as the citizen reference (refs #78)

The ACL now writes the domain registrationId as the zaak's identificatie on
POST /zaken, and exposes GET-through POST /zaken/reference to read a zaak's
identificatie back. Only the ACL touches ZGW (§8.1); the reference is the single
value shown on both the self-service confirmation and the openbaar register.

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

services/acl/Acl.Application/ZaakRequest.cs

@@ -6,4 +6,5 @@ public sealed record ZaakRequest(
     string VerantwoordelijkeOrganisatie,
     string Vertrouwelijkheidaanduiding,
     Uri Zaaktype,
-    DateOnly Startdatum);
+    DateOnly Startdatum,
+    string Identificatie);