Compatibility notes

Compilers

• Works with GCC/Clang/MSVC in C++23 mode.
• __SIZEOF_INT128__ fast-path used where supported.

ABI / headers

• Prefer including only the needed headers:
• umath/Int128.h
• umath/NumericFixed.h

Formatting

to_string() is intended to be stable across platforms (same output for same stored value).