feat(acl): wire BDD step bindings to AclService (refs #5)

Implement the step definitions for EenZaakOpenen.feature against the
existing AclService, with an in-memory gateway standing in for the OpenZaak
Zaken API. The scenario now passes: a domain registration is default-filled
into a ZaakRequest (ADR-0003) and the created zaak URL is returned.

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

tests/acceptance/Support/FixedClock.cs

@@ -0,0 +1,10 @@
+using Acl.Application;
+
+namespace Acceptance.Support;
+
+/// <summary>A clock pinned to a known date so <c>startdatum</c> is deterministic
+/// in scenarios (ADR-0003).</summary>
+public sealed class FixedClock(DateOnly today) : IClock
+{
+    public DateOnly Today { get; } = today;
+}