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>

.gitea/workflows/ci.yaml

@@ -68,6 +68,12 @@ jobs:
           name: event-subscriber-mutation-report
           path: services/event-subscriber/StrykerOutput/**/reports/mutation-report.html
           if-no-files-found: warn
+      - uses: https://github.com/actions/upload-artifact@v3
+        if: always()
+        with:
+          name: domain-mutation-report
+          path: services/domain/StrykerOutput/**/reports/mutation-report.html
+          if-no-files-found: warn
 
   # One stage for every check that needs the live stack. On the single self-hosted
   # runner jobs run sequentially, so booting OpenZaak once (instead of once per job)