Refactor Candidates

After TDD cycle, look for:

Duplication → Extract function/class
Long methods → Break into private helpers (keep tests on public interface)
Shallow modules → Combine or deepen — see DEEPENING.md in /codebase-design for dependency categories and seam discipline
Feature envy → Move logic to where data lives
Primitive obsession → Introduce value objects
Existing code the new code reveals as problematic