tmux refuses new-session from read-only clients ('client is
read-only'), so an active join for an -r invitee died under set -eu
with a raw tmux error. Detect the R flag on the caller's ACL line in
the existing preflight and fall back to the mirrored view.
tmux answers non-allowlisted users with 'access not allowed' on stderr
and exit 0 for every command (verified 3.3a/3.5a, two users), so
has-session reports any target as existing. party status claimed
uninvited group members were joined, party leave silently 'succeeded',
and party list printed bogus '0 attendee(s)' rows. New party_conn_state
(ok/unauthorized/dead) classifies by message content; list now shows
'invite-only (ask <host>)'.
