Wipes every rule-headed relation (the IDB) — leaves EDB facts and
rule definitions intact. Useful for inspecting the EDB-only
baseline or for forcing a clean re-saturation.
(dl-saturate! db)
(dl-clear-idb! db) ; ancestor relation now empty
(dl-saturate! db) ; re-derives ancestor from parents
2 new api tests verify IDB-wipe and EDB-preservation.