tmux_server:close/1

Retire this endpoint's operations and one-shot clients, preserving the borrowed tmux server. Returns ok only after their cleanup completes; incomplete cleanup remains an explicit error. Closing an already closed retained handle is idempotent. The initiator controls endpoint lifetime. Separate control attachments are invalidated and asked to retire; use tmux_control:close/1 before this call to verify those clients' cleanup too.