arch(acl): ADR-0005 adopt Stryker.NET for mutation testing (refs #47)

Record the decision to adopt Stryker.NET (pinned local tool, solution mode on
Acl.slnx) and to set the first repo-wide mutation baseline on the ACL: observed
95%, enforced break threshold 90%. Document the ratchet, local run, and report
location in the CI runbook; add the ADR to the docs nav.

Proposed in #51 (adr-proposal). Refs #47.

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

mkdocs.yml

@@ -26,6 +26,7 @@ nav:
       - "ADR-0002: Catalogus design": architecture/adr-0002-catalogus-design.md
       - "ADR-0003: ACL default-fill": architecture/adr-0003-default-fill.md
       - "ADR-0004: BDD framework": architecture/adr-0004-bdd-framework.md
+      - "ADR-0005: Mutation testing": architecture/adr-0005-mutation-testing.md
   - Working in Gitea: gitea-workflow.md
   - Runbooks:
       - CI: runbooks/ci.md