The custom parse_num floating-point implementation produced slightly
different results from the C++ compiler's own literal parsing (e.g.
1.1e-4 was off by a few ULPs). Replace it with std::from_chars which
is locale-independent and matches compiler precision exactly. Also
migrate integer parsing in buildInt from std::stoll/stoull to
std::from_chars, fix the JSON number parser to pass the full numeric
string to from_chars instead of splitting mantissa/exponent and using
std::pow, and update parse_string to use from_chars for arithmetic types.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
