test: add Infection mutation testing with enforced score and dashboard reporting

[turegjorup]

Summary

Adds Infection mutation testing to evaluate test suite sensitivity. Baseline run: 115 mutants, Covered Code MSI 80–84% against 100% line coverage, full run completes in ~14 seconds.

Features Added

• Infection as a dev dependency with infection.json5 config — minCoveredMsi: 78 and threads: max live in the config file as the single source of truth, so no command line flags are duplicated across Taskfile/CI
• task test:mutation task, also appended to task pr:actions
• CI job mutation-tests running with --logger-github (inline PR annotations for escaped mutants); GITHUB_* env vars are passed into the container so Infection's CI detection works
• Stryker dashboard reporting for develop (uses the STRYKER_DASHBOARD_API_KEY secret), feeding a new mutation score badge in the README
• README "Mutation Testing" section in the development docs

Files Changed

• composer.json - infection/infection dev dependency + allow-plugins entry
• infection.json5 (new) - Infection config incl. threshold and Stryker dashboard logger
• Taskfile.yml - test:mutation task, added to pr:actions
• .github/workflows/php.yaml - mutation-tests job
• README.md - badge + Mutation Testing section
• CHANGELOG.md - Unreleased/Added bullet
• .gitignore - generated infection.log/infection.html

Test Plan

• task test:mutation — passes locally (Covered MSI 84% ≥ 78%), dashboard upload correctly skipped outside CI
• task lint:markdown / task lint:yaml — pass
• CI on this PR should run the new mutation-tests job; dashboard upload and badge activate on first push to develop after merge

Notes

• The mutation score fluctuates a few points between runs (93–97 of 115 killed), so the threshold has headroom; follow-up PRs will kill the surviving mutants and ratchet minCoveredMsi above 90
• Badge shows "unknown" until the first develop run seeds the dashboard

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (c826902) to head (e7148d4).
⚠️ Report is 4 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##             develop       #44   +/-   ##
===========================================
  Coverage     100.00%   100.00%           
  Complexity        62        62           
===========================================
  Files              9         9           
  Lines            282       282           
===========================================
  Hits             282       282           

Flag	Coverage Δ
unittests	100.00% <ø> (ø)

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