Refactor Candidates

After TDD cycle, look for:

• Duplication → Extract function/class
• Long methods → Break into _ prefixed private helpers (keep tests on the public interface)
• Shallow modules → Combine or deepen
• Feature envy → Move logic to where data lives
• Primitive obsession → Introduce value objects as @dataclass(frozen=True)
• Existing code the new code reveals as problematic

Lint and Type Checks Are Non-Negotiable

All code must be warning- and error-free. Run just lint after every refactor step. This covers both ruff (style, imports, common errors) and basedpyright (type checking). Do not suppress warnings with ignore comments — fix the underlying issue. If you cannot resolve a type error, stop and ask.