Fix FreeBSD postfork child handler never being called: FreeBSD's libthr
calls _malloc_postfork in both parent and child (see freebsd-src
lib/libthr/thread/thr_fork.c), but jemalloc mapped it to the parent
handler only. Detect the child via getpid() and route to
jemalloc_postfork_child, which resets nthreads and rebuilds the
descriptor queue.
Remove the child_survivor_bytes vs pre_survivor_bytes comparison: on
macOS where jemalloc registers as the default zone, internal allocations
during the postfork handler (pthread_mutex_init) can inflate the
surviving thread's tcache.
Add double-fork test to verify prefork pid is refreshed correctly when a
child process forks again.
The orchestrator looks up the surviving descriptor via
tcache_postfork_arena_descriptor and threads it into
arena_postfork_child, eliminating arena's call into tcache. Also reset
cache_bin_array_descriptor_ql_mtx right before the queue rebuild it
protects.
