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/scoreboard.json

@@ -1,8 +1,8 @@
 {
   "lang": "datalog",
-  "total_passed": 206,
+  "total_passed": 208,
   "total_failed": 0,
-  "total": 206,
+  "total": 208,
   "suites": [
     {"name":"tokenize","passed":26,"failed":0,"total":26},
     {"name":"parse","passed":18,"failed":0,"total":18},
@@ -12,9 +12,9 @@
     {"name":"semi_naive","passed":8,"failed":0,"total":8},
     {"name":"negation","passed":10,"failed":0,"total":10},
     {"name":"aggregates","passed":18,"failed":0,"total":18},
-    {"name":"api","passed":15,"failed":0,"total":15},
+    {"name":"api","passed":17,"failed":0,"total":17},
     {"name":"magic","passed":21,"failed":0,"total":21},
     {"name":"demo","passed":18,"failed":0,"total":18}
   ],
-  "generated": "2026-05-08T10:04:46+00:00"
+  "generated": "2026-05-08T10:06:36+00:00"
 }