Implement

Implement via tests-first:
1) Create/extend failing tests under tests/**.
2) Minimal diffs in src/**; cite any Exa lines used.
3) Run ruff, mypy, pytest; iterate until green.
4) Keep changes ≤200 lines; split if larger.

> Token thrift:
- **Exa**: avoid broad/generic queries; include specific lib, function, or exact error text.