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/api.sx

@@ -213,3 +213,23 @@
                 (append! seen h))))
           (dl-rules db))
         seen))))
+
+;; Wipe every relation that has at least one rule (i.e. every IDB
+;; relation) — leaves EDB facts and rule definitions intact. Useful
+;; before a follow-up `dl-saturate!` if you want a clean restart, or
+;; for inspection of the EDB-only baseline.
+(define
+  dl-clear-idb!
+  (fn
+    (db)
+    (let ((rule-heads (dl-rule-head-rels db)))
+      (do
+        (for-each
+          (fn
+            (k)
+            (do
+              (dict-set! (get db :facts) k (list))
+              (dict-set! (get db :facts-keys) k {})
+              (dict-set! (get db :facts-index) k {})))
+          rule-heads)
+        db))))