test(acl): BDD acceptance scenario for opening a zaak (refs #5)

Introduce a Reqnroll.xUnit acceptance project under tests/acceptance/ and
the first Gherkin scenario driving S-04: the ACL default-fills the
ZGW-mandatory zaak fields (ADR-0003) from a domain registration payload.

Red: the scenario is discovered but has no step bindings yet, so it fails
with undefined steps. The green commit wires the bindings to AclService.

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

tests/acceptance/Acceptance.csproj

@@ -0,0 +1,22 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+    <IsPackable>false</IsPackable>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="coverlet.collector" Version="6.0.4" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.14.1" />
+    <PackageReference Include="Reqnroll.xUnit" Version="3.3.4" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="3.1.4" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\..\services\acl\Acl.Application\Acl.Application.csproj" />
+    <ProjectReference Include="..\..\services\acl\Acl.Infrastructure\Acl.Infrastructure.csproj" />
+  </ItemGroup>
+
+</Project>