Enforce code formatting with ruff format

[janhoy]

Summary

Now that #6 replaces pylint with ruff, the next step is to enforce consistent code formatting using `ruff format` — ruff's built-in formatter (compatible with Black's style).

Motivation

• The entire codebase can be formatted in one pass with `ruff format .`
• CI already runs `make lint`; adding `ruff format --check .` there enforces formatting on every PR
• A single tool (ruff) now covers linting and formatting — no separate Black/isort dependency needed
• Consistent formatting reduces noise in diffs and eliminates style debates in reviews

Proposed changes

1. Format the codebase — run `ruff format .` once to reformat all ~250 Python files
2. Enable the format check in `Makefile` — uncomment the `ruff format --check .` line in the `lint` target (the comment-placeholder was left there intentionally)
3. Document for developers in `CONTRIBUTING.md` or `DEVELOPER_GUIDE.md`:
   • Run `ruff format .` before committing (or configure your editor to do it on save)
   • `make lint` will fail in CI if files are not formatted
   • Optional: recommend a pre-commit hook or IDE integration (VS Code `ruff` extension, etc.)

Notes

• The formatting-only diff will be large (~190 files) but purely mechanical — no logic changes
• It is best submitted as a standalone PR so reviewers can ignore it or review with whitespace-diff disabled (`?w=1` in GitHub)
• The `[tool.ruff.format]` section can be added to `pyproject.toml` at that point (quote style, indent style)