test(acl): OpenZaak default-fills ZGW fields and posts to OpenZaak (refs #5)

Add the ACL skeleton (Application/Infrastructure/Api per §9) and failing
xUnit tests for the single operation: AclService.OpenZaakAsync must
default-fill bronorganisatie, verantwoordelijkeOrganisatie, startdatum
(clock) and vertrouwelijkheidaanduiding and delegate to the gateway; the
OpenZaakGateway must POST the zaak to OpenZaak with a Bearer JWT and the
default-filled body. Implementations throw NotImplementedException — red.

Also add ADR-0003 (default-fill strategy).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

services/acl/Acl.Application/IZaakGateway.cs

@@ -0,0 +1,8 @@
+namespace Acl.Application;
+
+/// <summary>Port to the ZGW Zaken API. Implemented in Infrastructure — the only
+/// code that talks to OpenZaak (ADR-0001).</summary>
+public interface IZaakGateway
+{
+    Task<Uri> OpenZaakAsync(ZaakRequest request, CancellationToken ct = default);
+}