test(domain): approving a registration advances INGEDIEND → INGESCHREVEN (refs #75)

Guards: an opened zaak is required and only a submitted registration can be
approved. Fails to compile — Registration.Approve() and RegistrationStatus.
Ingeschreven don't exist yet (red).

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

services/domain/Big.Tests/RegistrationTests.cs

@@ -87,4 +87,37 @@ public class RegistrationTests
         Assert.Contains("/zaken/abc", ex.Message);
         Assert.Contains("/zaken/other", ex.Message);
     }
+
+    [Fact]
+    public void Approving_a_registration_with_a_zaak_sets_it_ingeschreven()
+    {
+        var registration = Registration.Submit("123456782");
+        registration.AttachZaak(new Uri("http://openzaak/zaken/api/v1/zaken/abc"));
+
+        registration.Approve();
+
+        Assert.Equal(RegistrationStatus.Ingeschreven, registration.Status);
+    }
+
+    [Fact]
+    public void Approving_before_the_zaak_is_opened_is_rejected()
+    {
+        var registration = Registration.Submit("123456782");
+
+        var ex = Assert.Throws<InvalidOperationException>(() => registration.Approve());
+
+        Assert.Contains("no zaak", ex.Message, StringComparison.OrdinalIgnoreCase);
+        Assert.Equal(RegistrationStatus.Ingediend, registration.Status);
+    }
+
+    [Fact]
+    public void Approving_an_already_ingeschreven_registration_is_rejected()
+    {
+        var registration = Registration.Submit("123456782");
+        registration.AttachZaak(new Uri("http://openzaak/zaken/api/v1/zaken/abc"));
+        registration.Approve();
+
+        Assert.Throws<InvalidOperationException>(() => registration.Approve());
+        Assert.Equal(RegistrationStatus.Ingeschreven, registration.Status);
+    }
 }