Closes#94779. A turn that finished while the gateway socket was down never
replays its sessions.changed tick, so the open transcript stayed stale
until the user reopened the session. The gateway-open effect now also
requests one signature-gated tail of the active transcript on every
(re)connect (connection-scoped, so a plain session switch adds no read;
messaging transcripts already refresh on open via their own effect).
Reported-by: Kkkkkuro
6d061ede58