merged

NEWS; tuned description;

tuned proofs;

updated key bindings to execution range;

traverse node on change of "required" state;

keep memo_exec execution running, which is important to cancel goal forks eventually; run as nested worker task to keep main group valid, even after cancelation of removed subgroups; do not memoize interrupt, but absorb it silently in main execution; Goal.purge_futures: rough sanity check of group status;