refactor: implement composition pattern with Pin::Piece and update documentation

- Replace direct attribute storage with Pin::Piece composition for better maintainability
- Use PIN constants (NORMAL_STATE, NATIVE) instead of magic values
- Delegate type/side/state methods to internal Pin::Piece object
- Remove same_derivation? alias to reduce API entropy
- Rewrite complete README.md with updated architecture section and examples
- Improve code consistency and leverage proven PIN implementation

Author: cyril