Add Pest mutation testing quality gate

[erseco]

Summary

• add mutation testing with Pest's native mutation engine
• run mutation testing in a dedicated PHP 8.4 job without changing the parser's PHP 8.0 runtime requirement
• establish a 50% minimum mutation-score gate from the measured project baseline
• mutate only changed PHP source files in pull requests
• skip mutation execution when a PR has no changed PHP source
• run a full mutation pass across all covered source weekly
• allow manual full mutation runs with workflow_dispatch
• keep the existing 90% line-coverage gate as a separate quality signal
• document the mutation-testing policy, measured baseline and contributor expectations

Stack

This PR is stacked on #36 and targets quality/backward-compatibility.

Pull-request strategy

Changed PHP source paths are calculated from Git and supplied through Pest's supported --path option:

MUTATION_PATHS="$(git diff --name-only --diff-filter=AMR "origin/$BASE_REF...HEAD" -- 'src/*.php' 'src/**/*.php' | paste -sd, -)"

vendor/bin/pest \
  --mutate \
  --parallel \
  --path="$MUTATION_PATHS" \
  --covered-only \
  --ignore-min-score-on-zero-mutations \
  --min=50

Scheduled/manual full runs use --everything --covered-only --min=50.

Measured baseline

The initial full mutation run produced:

• mutation score: 53.66%
• tested mutants: 1,586
• untested mutants: 1,373
• timed-out mutants: 4

The initial CI floor is therefore set to 50%, slightly below the measured baseline. This starts the gate green while preventing regressions and allows the threshold to be ratcheted upward as mutation gaps are addressed.

Validation

• PR changed-source mutation check: passing
• full mutation baseline measured
• PHP 8.0–8.5 tests: passing
• coverage gate: passing
• Composer validation: passing
• PHPCS: passing
• PHPStan: passing

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.4%. Comparing base (a7d8e5b) to head (99d4cc4).

Additional details and impacted files

@@                       Coverage Diff                        @@
##             quality/backward-compatibility     #37   +/-   ##
================================================================
  Coverage                              92.4%   92.4%           
  Complexity                              801     801           
================================================================
  Files                                    30      30           
  Lines                                  2639    2639           
================================================================
  Hits                                   2441    2441           
  Misses                                  198     198           

Flag	Coverage Δ
unit	92.4% <ø> (ø)

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