Emacs Internal #03: Tagged Union, Tagged Pointer, and Poor Man's Inheritance

[Update 2026-03-17] Thanks to the Hacker News community (discussion id=47349780) for valuable architectural context, highlighting the C++ UB on raw pointers, Rust's Strict Provenance API, and modern LLVM CastInfo RTTI (specifically ndesaulniers , internet_points , tialaramex , trws , jcranmer , shadowgovt , and mshockwave ). Also, thanks to the Reddit community (specifically GuDzpoz ) for…