fix(ci): declare the lint surface so a ruff default change cannot break main

[vinicq]

main is red right now: test (3.13) failed and the other matrix legs were cancelled. This is the smallest change that fixes it, split out of #96 so a production-red branch is not waiting on a CI-hygiene discussion.

Cause

pyproject.toml had no [tool.ruff] section at all and declares ruff>=0.5 with no ceiling, so the lint surface was whatever ruff happened to default to. ruff 0.16 widened that default set. CI installs 0.16.2. No commit in this repo caused the failure.

Proved by removal, on this tree, with the version CI uses

State	ruff check src tests
with the select block	clean
stash it	67 errors

The findings are UP031, E402, BLE001, I001, SIM114, RUF100. None of them are new code. They are rules that were never selected and now are.

Why declare instead of cap

select = ["E4", "E7", "E9", "F"] is exactly what the repo was already being held to. A version cap (ruff>=0.5,<0.16) fixes today by freezing the tool while leaving the surface implicit, so the same break returns the day someone lifts the cap. Declaring it makes a future default widening a no-op and keeps ruff upgradable.

Same call as vinicq/falsegreen#148, where the identical drift left main with 106 findings.

Adopting the wider rule set is worth doing on its own merits, with the 67 findings triaged in a dedicated PR.

Verification

Full suite in a clean venv with pip install -e ".[dev]": 257 passed. (An earlier local run of mine showed failures; that was PYTHONPATH=src without an editable install, my environment and not this repo.)

Rollback

One hunk in one file. git revert restores a red main, which is the argument against it.