clarified worker_wait; clarified join_next/join_loop, with proper thread attributes to allow interrupting of join (e.g. via Lazy.force);

explicit headless mode for machines that lack X11 (e.g. lxbroy10);

more robust protocol command: purge removed execs asynchronously, to remain reactive despite problems to cancel "Command.run_process" in a situation of overrunning non-terminating tasks (see also 59f1591a11cb);

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