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/stryker-config.json

@@ -0,0 +1,15 @@
+{
+  "stryker-config": {
+    "solution": "Big.slnx",
+    "test-projects": ["Big.Tests/Big.Tests.csproj"],
+    "reporters": ["progress", "html"],
+    "mutate": [
+      "!**/OpenZaakJobPump.cs"
+    ],
+    "thresholds": {
+      "high": 95,
+      "low": 90,
+      "break": 90
+    }
+  }
+}