ractor: lock the owner when Port#closed? is queried from another Ractor

ractor_closed_port_p asserts the owning ractor's lock is held for foreign
access and reads sync.ports via st_lookup, but Ractor::Port#closed?
(ractor_port_closed_p) called it without the lock. From a foreign Ractor this
tripped the assertion and raced the owner's st_insert/st_delete on the ports
table. Take the ractor lock for foreign queries, like every other foreign
reader.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>