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.Tests/AclServiceTests.cs

@@ -0,0 +1,47 @@
+using Acl.Application;
+
+namespace Acl.Tests;
+
+public class AclServiceTests
+{
+    private sealed class FakeGateway : IZaakGateway
+    {
+        public ZaakRequest? Captured;
+        public Uri Result { get; } = new("http://openzaak/zaken/api/v1/zaken/abc");
+
+        public Task<Uri> OpenZaakAsync(ZaakRequest request, CancellationToken ct = default)
+        {
+            Captured = request;
+            return Task.FromResult(Result);
+        }
+    }
+
+    private sealed class FixedClock(DateOnly today) : IClock
+    {
+        public DateOnly Today { get; } = today;
+    }
+
+    [Fact]
+    public async Task Opening_a_zaak_default_fills_zgw_fields_and_returns_the_zaak_url()
+    {
+        var gateway = new FakeGateway();
+        var defaults = new AclDefaults
+        {
+            Bronorganisatie = "517439943",
+            VerantwoordelijkeOrganisatie = "517439943",
+            Vertrouwelijkheidaanduiding = "openbaar",
+            ZaaktypeUrl = new("http://openzaak/catalogi/api/v1/zaaktypen/big"),
+        };
+        var service = new AclService(gateway, defaults, new FixedClock(new DateOnly(2026, 6, 4)));
+
+        var url = await service.OpenZaakAsync(new DomainRegistration("123456782"));
+
+        Assert.Equal(gateway.Result, url);
+        var req = Assert.IsType<ZaakRequest>(gateway.Captured);
+        Assert.Equal("517439943", req.Bronorganisatie);
+        Assert.Equal("517439943", req.VerantwoordelijkeOrganisatie);
+        Assert.Equal("openbaar", req.Vertrouwelijkheidaanduiding);
+        Assert.Equal(defaults.ZaaktypeUrl, req.Zaaktype);
+        Assert.Equal(new DateOnly(2026, 6, 4), req.Startdatum);
+    }
+}