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/SubmitRegistrationTests.cs

@@ -21,6 +21,20 @@ public class SubmitRegistrationTests
         Assert.Equal("proc-42", saved.ProcessInstanceId);
         Assert.Equal(id, workflow.StartedFor);
         Assert.Null(saved.ZaakUrl);
+        // Persisted twice: once before starting the process, once after recording the instance id.
+        Assert.Equal(2, store.SaveCount);
+    }
+
+    [Fact]
+    public async Task Rejects_a_null_command_without_touching_the_store_or_workflow()
+    {
+        var store = new FakeRegistrationStore();
+        var workflow = new FakeWorkflowClient();
+        var handler = new SubmitRegistration(store, workflow);
+
+        await Assert.ThrowsAsync<ArgumentNullException>(() => handler.HandleAsync(null!));
+        Assert.Equal(0, store.SaveCount);
+        Assert.Null(workflow.StartedFor);
     }
 
     [Fact]