Task: Implement Hotfix: the macOS parser refuses the smallest subnormal

Make the round trip of the smallest denormal pass on macOS, and make the range check the header documents true on every platform.

• parse_double("5e-324") returns std::numeric_limits<double>::denorm_min().
• parse_double("1e-400") and parse_double("1e400") return std::nullopt.
• With de_DE.UTF-8 in force, parse_double("1.5") still parses and parse_double("1,5") is still refused.
• The conversion no longer passes through a stream, so libc++'s treatment of the range error cannot decide the answer.
• ores.platform.tests passes and the tree builds.

1. Reproduce the mechanism on Linux: show that the C conversion reports 5e-324 as a subnormal with ERANGE, which is the error libc++ turns into a stream failure.
2. Check the grammar by hand, convert in a C locale of the host's making, and classify the result by what the type can hold rather than by the stream's state.
3. Add the subnormal and the underflow to the suite, and demonstrate the locale independence.
4. Build the tree, run the platform suite and the suite.

• The macOS failure cannot be reproduced on Linux directly, because libstdc++ does not fail the extraction on the range error. What can be shown – and is shown above – is that the error is there on this platform too, and that the new code classifies by the value rather than by the stream.
• The header's promise was false in both directions before this change: it refused a subnormal on macOS and accepted an underflow everywhere else. The task makes the promise true rather than documenting the accident.
• __has_include(<xlocale.h>) guards the macOS header, so the file compiles whether or not the SDK still ships it; _locale_t and _strtod_l serve Windows, and newlocale=/=strtod_l the POSIX platforms.

parse_double checks the grammar itself and converts in a C locale of its own. The smallest denormal parses to denorm_min, 1e-400 and 1e400 are refused, and neither the program's locale nor libc++'s stream state decides the answer. The platform suite passes 328 assertions in 142 cases.