datalog: dl-rules-of relation-inspection helper (214/214)

(dl-rules-of db rel-name) → list of rules with head matching
the given relation name. Useful for tooling and debugging
("show me how this relation is derived") without exposing the
internal :rules list directly.

2 new api tests cover hit and miss cases.

lib/datalog/scoreboard.json

@@ -1,8 +1,8 @@
 {
   "lang": "datalog",
-  "total_passed": 212,
+  "total_passed": 214,
   "total_failed": 0,
-  "total": 212,
+  "total": 214,
   "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":17,"failed":0,"total":17},
+    {"name":"api","passed":19,"failed":0,"total":19},
     {"name":"magic","passed":22,"failed":0,"total":22},
     {"name":"demo","passed":18,"failed":0,"total":18}
   ],
-  "generated": "2026-05-08T10:15:09+00:00"
+  "generated": "2026-05-08T10:17:32+00:00"
 }