frozen_dataclass_sealable(refactor): Ensure explicit frozen=False

why: Explicitly setting frozen=False preserves inheritance flexibility

what: - Make dataclass(frozen=False) explicit in decorator implementation - Fix type errors with proper mypy guards - Add warning when frozen=True is incorrectly specified - Keep AttributeError for consistency with existing tests - Fix line length issues and improve error messages

refs: Addresses core issues with inheritance from mutable base classes

Author: tony