Apply ruff format to src and tests

[szymoniwacz]

Summary

• One-shot ruff format on src/ and tests/ (behavior-preserving; AST identical)
• CI now runs the documented quality suite: ruff format --check src tests, ruff check ., mypy ., and pytest
• Docs updated (README, stack profile, publishing, CHANGELOG)

Test plan

• Local: AST identical for all 18 changed files vs main
• Local: ruff format --check src tests, ruff check ., mypy ., pytest (280)
• Local: format idempotent; CLI smoke (diffrat --help)
• Local: Python 3.13 suite green (CI uses 3.11)
• CI validate green on this PR after the CI gates commit