datalog: dl-clear-idb! helper (208/208)

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.

lib/datalog/tests/api.sx

@@ -213,6 +213,30 @@
           "?- p(X), q(X).")
         (list {:X 2} {:X 3}))
 
+      ;; dl-clear-idb!: wipe rule-headed relations.
+      (dl-api-test! "dl-clear-idb! wipes IDB"
+        (let
+          ((db (dl-program
+                 "parent(a, b). parent(b, c).
+                  ancestor(X, Y) :- parent(X, Y).
+                  ancestor(X, Z) :- parent(X, Y), ancestor(Y, Z).")))
+          (do
+            (dl-saturate! db)
+            (dl-clear-idb! db)
+            (len (dl-relation db "ancestor"))))
+        0)
+
+      (dl-api-test! "dl-clear-idb! preserves EDB"
+        (let
+          ((db (dl-program
+                 "parent(a, b). parent(b, c).
+                  ancestor(X, Y) :- parent(X, Y).")))
+          (do
+            (dl-saturate! db)
+            (dl-clear-idb! db)
+            (len (dl-relation db "parent"))))
+        2)
+
       ;; dl-eval-magic — routes single-goal queries through
       ;; magic-sets evaluation.
       (dl-api-test-set! "dl-eval-magic ancestor"