An observer can now run a pane at its own size (ClientMsg::Lease, feature
size-lease). The pty and every observer go to that size. The controller
keeps its grid, its resizes are remembered rather than applied, and the
pane goes back to the last of them when the lease ends: the observer lets
go, its connection closes, or the controller takes it back.
A controller hears about leases only after asking (Watch), since an older
client cannot decode DaemonMsg::Lease. The desktop asks on every attach,
spawn and relink where the daemon advertises the feature, locally or
through the host hello for remote workspaces, and shows the pane as in
use on the phone with a Take Back button.