datalog: dl-summary inspection helper (173/173)

Returns {<rel-name>: tuple-count} for relations with any tuples
or that are rule-headed (so empty IDB shows as :rel 0 rather than
disappearing). Skips placeholder entries from internal
dl-ensure-rel! calls. 4 tests cover basic, empty IDB, mixed
EDB+IDB, and empty-db cases.

lib/datalog/scoreboard.json

@@ -1,13 +1,13 @@
 {
   "lang": "datalog",
-  "total_passed": 169,
+  "total_passed": 173,
   "total_failed": 0,
-  "total": 169,
+  "total": 173,
   "suites": [
     {"name":"tokenize","passed":26,"failed":0,"total":26},
     {"name":"parse","passed":18,"failed":0,"total":18},
     {"name":"unify","passed":28,"failed":0,"total":28},
-    {"name":"eval","passed":18,"failed":0,"total":18},
+    {"name":"eval","passed":22,"failed":0,"total":22},
     {"name":"builtins","passed":19,"failed":0,"total":19},
     {"name":"semi_naive","passed":8,"failed":0,"total":8},
     {"name":"negation","passed":10,"failed":0,"total":10},
@@ -15,5 +15,5 @@
     {"name":"api","passed":11,"failed":0,"total":11},
     {"name":"demo","passed":15,"failed":0,"total":15}
   ],
-  "generated": "2026-05-08T09:27:29+00:00"
+  "generated": "2026-05-08T09:30:37+00:00"
 }