add ctor extensions, fast properties, undefined-as-default, and clang-format

[MangelSpec]

Summary

• add ctor factory and multi-dispatch overload support to class_
• add fast property accessors (get_option/set_option pattern)
• add overload.hpp for runtime function overloading with defaults
• treat explicit undefined as "use the default" to match JS native behavior
• apply clang-format to all source files
• add clang-format-19 check to CI workflow