A channel's request to enter / leave the FOCUS layout, emitted on
RealtimeSessionService.ChannelFocus$ when a plugin calls its context's
SetFocusMode. The overlay shell subscribes: it collapses/restores the main call column
and remembers which channel holds focus (so the floating pill's "exit" can be routed
back via BaseRealtimeChannelClient.RequestFocusExit).
A channel's request to enter / leave the FOCUS layout, emitted on RealtimeSessionService.ChannelFocus$ when a plugin calls its context's
SetFocusMode. The overlay shell subscribes: it collapses/restores the main call column and remembers which channel holds focus (so the floating pill's "exit" can be routed back via BaseRealtimeChannelClient.RequestFocusExit).