test(domain): Registration aggregate invariants (refs #6)

Failing unit tests for the Registration aggregate root: a submission starts
in INGEDIEND carrying its bsn, an empty/whitespace/null bsn is rejected, the
started process-instance id is remembered, and attaching the zaak the ACL
opened records its URL idempotently (a conflicting URL is rejected) while the
status stays INGEDIEND.

The aggregate is a stub (no-op mutators, empty bsn) so the tests compile and
fail on their assertions; the green commit implements the invariants.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

services/domain/Big.Tests/Big.Tests.csproj

@@ -0,0 +1,25 @@
+<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="xunit" Version="2.9.3" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="3.1.4" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <Using Include="Xunit" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\Big.Domain\Big.Domain.csproj" />
+  </ItemGroup>
+
+</Project>

services/domain/Big.Tests/RegistrationTests.cs

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+using Big.Domain;
+
+namespace Big.Tests;
+
+public class RegistrationTests
+{
+    [Fact]
+    public void Submitting_a_registration_starts_in_ingediend()
+    {
+        var registration = Registration.Submit("123456782");
+
+        Assert.Equal(RegistrationStatus.Ingediend, registration.Status);
+        Assert.Equal("123456782", registration.Bsn);
+        Assert.NotEqual(Guid.Empty, registration.Id.Value);
+        Assert.Null(registration.ZaakUrl);
+        Assert.Null(registration.ProcessInstanceId);
+    }
+
+    [Theory]
+    [InlineData("")]
+    [InlineData("   ")]
+    [InlineData(null)]
+    public void Submitting_without_a_bsn_is_rejected(string? bsn)
+        => Assert.Throws<ArgumentException>(() => Registration.Submit(bsn!));
+
+    [Fact]
+    public void Recording_the_started_process_keeps_its_instance_id()
+    {
+        var registration = Registration.Submit("123456782");
+
+        registration.RecordProcessStarted("proc-42");
+
+        Assert.Equal("proc-42", registration.ProcessInstanceId);
+    }
+
+    [Fact]
+    public void Attaching_a_zaak_records_its_url_and_keeps_the_status_ingediend()
+    {
+        var registration = Registration.Submit("123456782");
+        var zaak = new Uri("http://openzaak/zaken/api/v1/zaken/abc");
+
+        registration.AttachZaak(zaak);
+
+        Assert.Equal(zaak, registration.ZaakUrl);
+        Assert.Equal(RegistrationStatus.Ingediend, registration.Status);
+    }
+
+    [Fact]
+    public void Attaching_a_null_zaak_is_rejected()
+    {
+        var registration = Registration.Submit("123456782");
+
+        Assert.Throws<ArgumentNullException>(() => registration.AttachZaak(null!));
+    }
+
+    [Fact]
+    public void Re_attaching_the_same_zaak_is_idempotent()
+    {
+        var registration = Registration.Submit("123456782");
+        var zaak = new Uri("http://openzaak/zaken/api/v1/zaken/abc");
+
+        registration.AttachZaak(zaak);
+        registration.AttachZaak(zaak);
+
+        Assert.Equal(zaak, registration.ZaakUrl);
+    }
+
+    [Fact]
+    public void Attaching_a_conflicting_zaak_is_rejected()
+    {
+        var registration = Registration.Submit("123456782");
+        registration.AttachZaak(new Uri("http://openzaak/zaken/api/v1/zaken/abc"));
+
+        Assert.Throws<InvalidOperationException>(
+            () => registration.AttachZaak(new Uri("http://openzaak/zaken/api/v1/zaken/other")));
+    }
+}