+- **CI:** added Infection mutation testing gate, layered on top of the line-coverage gate. New `infection/infection ^0.32.7` dev dependency, `infection.json5` config (`@default` mutator profile, `src/` source scope, fixtures stay out via PHPUnit's existing `<source>` block, `--testsuite=Rules`), and two new composer scripts: `mutation` (local, `--threads=max --show-mutations` for inspecting escaped mutants) and `mutation:ci` (CI: `--threads=4 --no-progress --logger-github --min-msi=75 --min-covered-msi=75` — GitHub annotations on escaped mutants surface inline in PR diffs). Two new CI steps after the coverage gate: **Mutation testing** and **Upload mutation report** (per-leg `infection-php-${{ matrix.php }}` artifact, 14-day retention). `composer config allow-plugins.infection/extension-installer true` was set to permit the framework-adapter installer plugin. **Initial thresholds: 75% MSI and 75% Covered Code MSI** — measured baseline is 78.5% MSI (241 killed / 307 mutants, 100% Mutation Code Coverage), set 3.5 percentage points lower to absorb mutator-shape fluctuation on equivalent code. Same shape as the line-coverage gate: lock in current state, audit gaps, ratchet upward. The 22% surviving-mutant population audits as a mix of (a) genuinely-equivalent mutants the issue itself anticipated — `mb_stripos` ↔ `stripos` on PSR-4 ASCII-only class names in `LogRule`, defensive guard inversions (`LogicalNot`/`IfNegation`) on early returns that filter the same nodes by either condition — and (b) genuinely-uncovered branch logic that warrants new fixtures. A follow-up issue will audit each survivor, kill where realistic, `@infection-ignore-for-mutator`-annotate where equivalent, and ratchet thresholds to the issue's target of 80% MSI / 90% Covered Code MSI. Versioning: none (pure CI/test-infra, no consumer-visible behaviour; `infection` is `require-dev` only). Resolves issue #10.
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