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/tests/api.sx

@@ -213,6 +213,20 @@
           "?- p(X), q(X).")
         (list {:X 2} {:X 3}))
 
+      ;; dl-rules-of: rules with head matching a relation name.
+      (dl-api-test! "dl-rules-of count"
+        (let
+          ((db (dl-program
+                 "p(1). q(X) :- p(X). r(X) :- p(X). q(2).")))
+          (len (dl-rules-of db "q")))
+        1)
+
+      (dl-api-test! "dl-rules-of empty"
+        (let
+          ((db (dl-program "p(1). p(2).")))
+          (len (dl-rules-of db "q")))
+        0)
+
       ;; dl-clear-idb!: wipe rule-headed relations.
       (dl-api-test! "dl-clear-idb! wipes IDB"
         (let