guard is_space against negative signed code units#395
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is_space gates its space_lut lookup with
c < 256, but that comparison is signed whenever UC is a signed type, and on Linux/macOS wchar_t is a signed 32-bit int. A negative code unit therefore passes the guard and is then narrowed with uint8_t(c), so a wide unit whose low byte is 0x20 but whose full value is negative (e.g. 0xFFFFFF20) is classified as whitespace. With skip_white_space enabled, from_chars silently skips that leading unit and parses the remainder, so an input that should be rejected comes back as a successful parse.Comparing the value through its unsigned type before the lookup keeps the guard honest for every code unit and leaves char/char16_t/char32_t behaviour unchanged. ch_to_digit a few lines below already takes this precaution for the same signed-char reason, so this brings is_space into line rather than asking callers to pre-sanitise. I added a regression case alongside the existing wide-char tests; it fails on the old code (the bogus prefix is accepted) and passes with the unsigned compare.