Report the thread's affinity mask as a 32-byte FreeBSD cpuset_t, with
FreeBSD 11's errors: ERANGE for any other set size, ESRCH for a null
thread and EFAULT for a null set.
EVO-PLAYER (PPSA99039 v0.11.0) imports it (Jb2uGFMr688) and calls it
once with a 32-byte set on pthread_self().
Returns the CPU mask threads get by default (DEFAULT_THREAD_AFFINITY,
0x1FFF), the value itself rather than an error code.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>