io.github.libtmux.Server.Server.killServer

Deliberately explicit and deliberately not what #close() does: closing a client is not a reason to end everyone else's sessions.

Killing a server that has already gone is not a failure. Which is harder to detect than it sounds: tmux answers a doomed request with no server running most of the time, but with server exited unexpectedly when it reaches a socket whose server is still exiting — on every release from 3.3a onwards, and on one in five attempts on 3.7. So the postcondition is checked rather than the wording, which has changed before and says nothing a second look cannot answer better.