ControlModeSubscriptions

Description#

tmux answers refresh-client -B with %subscription-changed whenever the named format's value changes. There is no typed surface for building that command otherwise, so a caller renders it by hand - and tmux 3.8 tightened session-scoped targets: the session id form that fired on every earlier release (name:$0:format) silently stops firing, with no error, while an empty session field (name::format) fires on every released version. This always renders the empty form, so a session-scoped subscription is never reachable the wrong way.