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.Infrastructure/Acl.Infrastructure.csproj

@@ -0,0 +1,13 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <ItemGroup>
+    <ProjectReference Include="..\Acl.Application\Acl.Application.csproj" />
+  </ItemGroup>
+
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+  </PropertyGroup>
+
+</Project>

services/acl/Acl.Infrastructure/OpenZaakGateway.cs

@@ -0,0 +1,10 @@
+using Acl.Application;
+
+namespace Acl.Infrastructure;
+
+/// <summary>The only code that talks to OpenZaak's Zaken API (ADR-0001).</summary>
+public sealed class OpenZaakGateway(HttpClient http, OpenZaakOptions options) : IZaakGateway
+{
+    public Task<Uri> OpenZaakAsync(ZaakRequest request, CancellationToken ct = default)
+        => throw new NotImplementedException();
+}

services/acl/Acl.Infrastructure/OpenZaakOptions.cs

@@ -0,0 +1,9 @@
+namespace Acl.Infrastructure;
+
+/// <summary>Connection + credential config for OpenZaak's ZGW APIs.</summary>
+public sealed class OpenZaakOptions
+{
+    public required Uri BaseUrl { get; init; }
+    public required string ClientId { get; init; }
+    public required string Secret { get; init; }
+}

services/acl/Acl.Infrastructure/SystemClock.cs

@@ -0,0 +1,8 @@
+using Acl.Application;
+
+namespace Acl.Infrastructure;
+
+public sealed class SystemClock : IClock
+{
+    public DateOnly Today => DateOnly.FromDateTime(DateTime.UtcNow);
+}

services/acl/Acl.Infrastructure/ZgwToken.cs

@@ -0,0 +1,29 @@
+using System.Security.Cryptography;
+using System.Text;
+using System.Text.Json;
+
+namespace Acl.Infrastructure;
+
+/// <summary>Mints a ZGW (vng-api-common) JWT: HS256 over the standard claims.</summary>
+internal static class ZgwToken
+{
+    public static string Mint(string clientId, string secret)
+    {
+        var header = B64Url(JsonSerializer.SerializeToUtf8Bytes(new { alg = "HS256", typ = "JWT" }));
+        var payload = B64Url(JsonSerializer.SerializeToUtf8Bytes(new
+        {
+            iss = clientId,
+            iat = DateTimeOffset.UtcNow.ToUnixTimeSeconds(),
+            client_id = clientId,
+            user_id = "acl",
+            user_representation = "acl",
+        }));
+        var signingInput = $"{header}.{payload}";
+        using var hmac = new HMACSHA256(Encoding.UTF8.GetBytes(secret));
+        var signature = B64Url(hmac.ComputeHash(Encoding.UTF8.GetBytes(signingInput)));
+        return $"{signingInput}.{signature}";
+    }
+
+    private static string B64Url(byte[] bytes) =>
+        Convert.ToBase64String(bytes).TrimEnd('=').Replace('+', '-').Replace('/', '_');
+}