beautysh

test: cover orphan esac and _is_case_pattern branches; tighten case assertion

refactor: FormatResult return type; parser/transformers to free functions; move stderr to cli

test: fixture idempotency, bash -n validation, fix tautological property assertions

chore: enable mypy strict flags, ruff B/SIM/RUF, branch coverage, py.typed

chore(nix): add python version matrix checks; mypy pre-commit always full package