test: add mutation testing with mutmut

[zurfjereluhmie]

• Add mutmut>=3.5 as a dev dependency for mutation testing
• Configure [tool.mutmut] in pyproject.toml:
  • Targets src/evalwire/ for mutation
  • Excludes __init__.py and cli.py from mutation (no meaningful logic)
  • Excludes hypothesis property tests and integration tests from mutation runs (fork-incompatible / slow)
• Add make mutate target to run mutation testing
• Add mutants/ to .gitignore

Usage

# Run mutation testing on the full codebase
make mutate

# Run on a specific module
uv run mutmut run "evalwire.evaluators.contains*"

# Browse results interactively
uv run mutmut browse