test(domain): mutation baseline 90 (achieved 97.7%) + CI/Makefile wiring (refs #6)

Stryker.NET config for the domain service (break 90, the repo's ratchet floor),
excluding the OpenZaakJobPump hosted-shell from mutation. Hardened the unit tests
to kill survivors — Basic-credential value, variable types, null/failure response
paths, option defaults, guard clauses, save counts and log output — leaving only
two documented equivalent mutants (Stryker-disabled). make mutation runs the domain
ratchet and CI uploads its report alongside the others.

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

services/domain/Big.Tests/InMemoryRegistrationStoreTests.cs

@@ -33,4 +33,12 @@ public class InMemoryRegistrationStoreTests
         Assert.NotNull(loaded);
         Assert.Equal("http://openzaak/zaken/api/v1/zaken/abc", loaded.ZaakUrl!.ToString());
     }
+
+    [Fact]
+    public async Task Saving_a_null_registration_is_rejected()
+    {
+        var store = new InMemoryRegistrationStore();
+
+        await Assert.ThrowsAsync<ArgumentNullException>(() => store.SaveAsync(null!));
+    }
 }