Replace the strtoull + errno + cast-back dance in get_header_value_u64
with the existing hand-written detail::from_chars, which reports
result_out_of_range at size_t width. This detects the 32-bit truncation
case directly (instead of via a separate cast-back comparison), drops
the reliance on the global errno, and keeps the parsing locale-
independent and consistent with the rest of the codebase.
3adc525cec
