Problem

ruff check on main reports thousands of findings — reproduced on a clean checkout, pre-existing drift, not caused by any recent PR.

Root cause: pyproject.toml declares ruff>=0.1.0 (unpinned) with a [tool.ruff.lint] ignore list but no explicit select. The ignore list (E402, F841) only makes sense for the classic pyflakes/pycodestyle set — without a pinned select, newer ruff resolves its own broader default, pulling in categories (datetimez, simplify, pyupgrade, bugbear...) never intended by this repo, on top of never-enforced import-sort (I001) findings across the codebase.

Fix

• Added explicit select = ["E4", "E7", "E9", "F", "I"] to [tool.ruff.lint] — restores the classic error/pyflakes/import-sort scope the ignore list was written for, insulated from ruff version drift going forward.
• Ran ruff check --fix after pinning select, so only import reordering was applied (103 files) — no pyupgrade (Optional[X] → X | None) or Ruff-specific rewrites (f-string !s/!r conversion, __all__ sorting) snuck in. (An earlier attempt at this fix ran --fix before scoping select, which picked up those extra categories — redone here to be strictly import-sort only.)
• black --check was already clean on main — no reformatting needed.

Verification

• ruff check src/ tests/ → clean
• black --check src/ tests/ → clean, 0 files changed
• pytest tests/unit → 321 passed, unchanged
• Diff contains zero Optional[X] -> X | None or f-string conversion rewrites (verified via grep)

Purely mechanical import-sort — no behavioral or type-annotation changes.