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/api.sx

@@ -198,6 +198,24 @@
                 (dl-magic-query db (first qbody)))
               (else (dl-query db qbody)))))))))
 
+;; List rules whose head's relation matches `rel-name`. Useful for
+;; inspection ("show me how this relation is derived") without
+;; exposing the internal `:rules` list.
+(define
+  dl-rules-of
+  (fn
+    (db rel-name)
+    (let ((out (list)))
+      (do
+        (for-each
+          (fn
+            (rule)
+            (when
+              (= (dl-rel-name (get rule :head)) rel-name)
+              (append! out rule)))
+          (dl-rules db))
+        out))))
+
 (define
   dl-rule-head-rels
   (fn