tooling: pin the ruff rule selection and move CI to 0.16.1

[dcj]

Closes #39. Also adds .DS_Store to .gitignore.

What the investigation actually found

The issue as filed assumed the 0.15.21 -> 0.16.x delta was limited to Markdown formatting. It is not. Ruff 0.16 also widened its default lint selection, so on an unchanged codebase:

	ruff check .	ruff format --check .
0.15.21 (old pin)	clean	clean
0.16.1	381 errors	5 files would be reformatted

The 381 are real source, not Markdown: 194 UP045 (Optional[X] -> X | None), 44 UP006 (typing.List -> list), 38 LOG015, 17 BLE001, and a long tail. The repo declared only ignore, never select, so it silently inherited whatever the installed ruff considered default.

The fix

Declare the selection explicitly so it stops depending on ruff's defaults:

[tool.ruff]
line-length = 120
extend-exclude = ["*.md"]

[tool.ruff.lint]
select = ["E4", "E7", "E9", "F"]
ignore = ["E402", "E721"]

That is exactly ruff's historical default set, so this preserves current behaviour rather than changing it. Verified clean on both versions with zero source changes:

ruff@0.16.1  check -> All checks passed!   format --check -> 31 files already formatted
ruff@0.15.21 check -> All checks passed!   format --check -> 31 files already formatted

548 passed.

Deliberately not done

Adopting the 381 violations. That is a real codebase modernization touching type annotations throughout, with requires-python >= 3.10 implications, and it deserves its own reviewable diff rather than riding in on a pin bump. Widening select is now an explicit act.