[tonyfettes]

Summary

The macOS spawn path called the private __fork (weak-imported, returning ENOSYS if absent) to skip pthread_atfork handlers. That isn't needed:

• Nothing we link registers pthread_atfork handlers. The MoonBit runtime (libmoonbitrun.o) statically links mimalloc, but as a non-overriding build: it only serves mi_malloc for MoonBit objects, does not replace malloc or register a malloc zone, and neither it nor the linked test binaries import pthread_atfork (checked with nm -u on moon 0.1.20261023). So the only handlers __fork skipped were libSystem's own, and those are what keep the child's libc state consistent (malloc zone locks taken around the fork, mach ports reset in the child).
• The forked child only makes plain syscalls (sigaction, setsid, ioctl(TIOCSCTTY), dup2, chdir, execve, nanosleep) and never allocates (neither through malloc nor mimalloc), so plain fork() is sufficient.

We still have to fork rather than posix_spawn on macOS (XNU runs file actions before POSIX_SPAWN_SETSID, so the child can't acquire the pty as its controlling terminal); this only changes which fork is called. All unix platforms now share the same fork() path that Linux already used.

Testing

• macOS (Darwin 25.6): moon test passed 26/26 in 13 consecutive runs, no hangs.
• Linux is unchanged (it already used fork()).