Client.close

Retire the generation and close owned control clients and descriptors. Reject new submit and subscribe calls as soon as closing starts. Concurrent calls await the same teardown result. Later calls retain a teardown failure instead of treating the closed flag as success. Each helper gets 200 ms of TERM grace, followed by KILL if needed and reaping. Other fibers can run during that wait. The grace is per helper; it does not bound scheduling, filesystem or uninterruptible kernel work. No signal targets the daemon. tmux's exit-unattached and exit-empty policies can end the daemon after its clients or sessions disappear.