parse_num_ used std::from_chars for both integers and doubles, but the
floating-point from_chars overload is =deleted in Apple clang's libc++, so
the macOS build failed to compile (parse.cpp:136, instantiated for double).
Split parse_num_ with `if constexpr`: integers keep std::from_chars
everywhere; on macOS, doubles parse via strtod_l with a cached "C" locale
(locale-independent, restoring the pre-charconv Apple path). libstdc++ and
the MSVC STL have working float from_chars and are left unchanged.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Thomas Krijnen