test(generate): lock in sendmsg write byte-count classification

Audit of sendmsg(2) found the tracing implementation already correct:
enter is an fd_event with fd=args[0] (the kernel tracepoint first field
is 'int fd'), family is Network, and the exit is WRITE_CLASSIFIED so the
bytes-sent return value is counted as written, consistent with the
send/sendto/write siblings and distinct from recvmsg (read side) and the
deferred sendmmsg batch variant.

Add TestClassifySendmsgWriteByteCount as a lock-in regression test
pinning KindFd + FamilyNetwork + WRITE_CLASSIFIED, with recvmsg and
sendmmsg contrast cases to guard against read/write and batch
misclassification.

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

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