Module errno_compat

internal errno-accessor compatibility shim.

Not part of the public std:: API — included by the handful of std/ files that need 'errno' as a plain lvalue (convert.sc, errno.sc). Every libc here exposes 'errno' as a thread-local value reached through a per-libc accessor function, never a plain extern global:

- macOS/BSD: '__error()' — the documented macOS <errno.h> expansion.
- glibc:     '__errno_location()'.
- MSVC/UCRT: '_errno()' (returns 'errno_t*', itself just 'int*').

This header hides that split behind one 'errno' object-like macro (fine in safe mode — it isn't function-like, just an object-like macro whose replacement text happens to contain a call), usable as a plain lvalue exactly like C's own <errno.h> macro: 'errno = 0;', 'if (errno == ...)'.