test(generate): lock in recvmsg READ_CLASSIFIED audit

recvmsg(2) returns the number of bytes received, so its exit must be
READ_CLASSIFIED (bytes counted as read), matching recvfrom/recv/read/readv.
Audit confirmed the implementation is already correct: enter=KindFd off
the first 'fd' field (sockfd at args[0]), family=Network, exit=
READ_CLASSIFIED. Add a dedicated lock-in test mirroring the prior sendmsg
audit, with contrast cases guarding the easy mistakes: sendmsg is the
write-side sibling (WRITE_CLASSIFIED) and recvmmsg is the batch variant
whose scalar return is a message count, deferred to UNCLASSIFIED.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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