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>

docs/architecture/adr-0005-mutation-testing.md

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+# ADR-0005: Stryker.NET for mutation testing, baseline on the ACL
+
+- **Status:** Accepted
+- **Date:** 2026-06-25
+- **Deciders:** Respellion engineering
+- **Relates to:** S-04b (#47); proposed in #51; supports CLAUDE.md §5 (mutation ratchet) and §3 (Definition of Done)
+
+## Context
+
+CLAUDE.md §5 mandates Stryker on every PR with a **ratchet**: CI fails on a regression
+below the established baseline, and the baseline only ever moves up. §3 lists "mutation
+(ratchet)" as a Definition-of-Done gate for **every** slice. Yet no baseline existed — so,
+strictly, no slice could satisfy that gate. S-04b establishes it.
+
+The ACL is the natural place to set the first baseline: it is the first service with real
+branching logic — `OpenZaakGateway` (HTTP contract, geo CRS headers, error handling),
+`ZgwToken` (HS256 JWT minting), and the `AclService` default-fill mapping. We need a tool
+that:
+
+- mutates C# and runs the existing xUnit suite per mutant,
+- is reproducible (same version locally and in CI, no global install),
+- understands this repo's `.slnx` solution format (used repo-wide),
+- emits a break threshold CI can gate on.
+
+## Decision
+
+**Use [Stryker.NET](https://stryker-mutator.io/docs/stryker-net/) (`dotnet-stryker`),
+pinned as a local dotnet tool**, configured in solution mode against `Acl.slnx`.
+
+- Pinned in `.config/dotnet-tools.json` (v4.15.0); `dotnet tool restore` makes
+  `make mutation` reproducible from a fresh clone, locally and in CI — no global install.
+- **Solution mode** (`stryker-config.json` → `solution: Acl.slnx`) mutates the two projects
+  under test (`Acl.Application`, `Acl.Infrastructure`); `Acl.Api` is untested and skipped.
+  Stryker 4.15 reads `.slnx` directly, so no throwaway `.sln` shim is needed.
+- A `mutation` make target runs it; it is wired into `make ci` and a parallel Gitea Actions
+  `mutation` job, keeping `make ci` an exact mirror of the pipeline.
+
+**Baseline:** writing S-04b's tests surfaced that the ACL suite was thin — the initial
+score was **35%** (survivors: unasserted CRS headers, null guards, error paths, and JWT
+claims). Those tests were strengthened (killing the mutants honestly rather than lowering
+the bar), raising the score to **95%**. The enforced `break` threshold is set to **90%** —
+one-mutant headroom over the ~20-mutant surface, since a single mutant is ≈5%.
+
+## Consequences
+
+- **Positive:** test *strength* is gated, not just coverage; the ratchet protects the ACL's
+  ZGW contract logic; the baseline is repo-wide and ratchets upward per §5.
+- **Cost:** a new dependency (`dotnet-stryker`) and a slower CI job than unit tests (~25 s on
+  the small ACL). Pinned + tool-restored, so reproducible.
+- **One accepted survivor:** a mutation of the empty-response *exception message string*.
+  Asserting exception message text is brittle and the behaviour (type + control flow) is
+  unchanged — treated as an equivalent mutant, not a test gap.
+- **Commitment:** later slices ratchet the threshold up deliberately, never down (§5). New
+  services add their own mutation run as they gain branching logic (BFF, Domain, …).
+- **Replaceable by:** no realistic .NET alternative — Stryker.NET is the tool §5 already
+  names; the fallback is no mutation testing, which §5 forbids.
+
+## Alternatives considered
+
+- **Global `dotnet tool install -g`** — rejected: not reproducible/pinned per clone; the
+  local manifest gives every checkout and the CI runner the same version.
+- **Mutate the whole `register-referentie.slnx`** — rejected for this slice: scopes the
+  baseline to services with no logic yet (BFF skeleton), diluting the signal. Each service
+  opts in as it gains logic.
+- **Application-only scope** — rejected: would leave `Acl.Infrastructure`'s HTTP/JWT logic —
+  the riskiest code — unguarded by the ratchet.
+- **Coverage gate instead of mutation** — rejected: line coverage does not measure whether
+  tests would *catch* a regression; that is the whole point of §5.