Validates and plans an atomic fan-out: N independent put legs that must ride in one AVM group (all-or-nothing delivery).
1..MAILBOX_MAX_FANOUT_LEGS legs
One plan per leg, in input order
MailboxFanoutLimitError if more than 8 legs (or zero) are given
Validates and plans an atomic fan-out: N independent put legs that must ride in one AVM group (all-or-nothing delivery).