Linting improvements roadmap

This note proposes incremental linting improvements that increase signal without creating large one-off cleanup projects.

1. Expand Ruff to include import and modernization checks

Current Ruff usage is focused on parser/runtime failures (E9, F823). Add low-noise rule sets in phases:

1. I (import sorting)
2. UP (safe Python upgrades)
3. B (selected bugbear checks)

Start in non-blocking report mode first (--exit-zero) so maintainers can prioritize real issues before deciding whether to enforce.