datalog: dl-eval-magic single-call magic-sets entry (206/206)

Symmetric to dl-eval but routes single-positive-literal queries
through dl-magic-query for goal-directed evaluation. Multi-literal
query bodies fall back to standard dl-query (magic-sets is wired
for single goals only).

  (dl-eval-magic source-string "?- ancestor(a, X).")

1 new api test.

lib/datalog/tests/api.sx

@@ -213,6 +213,16 @@
           "?- p(X), q(X).")
         (list {:X 2} {:X 3}))
 
+      ;; dl-eval-magic — routes single-goal queries through
+      ;; magic-sets evaluation.
+      (dl-api-test-set! "dl-eval-magic ancestor"
+        (dl-eval-magic
+          "parent(a, b). parent(b, c).
+           ancestor(X, Y) :- parent(X, Y).
+           ancestor(X, Z) :- parent(X, Y), ancestor(Y, Z)."
+          "?- ancestor(a, X).")
+        (list {:X (quote b)} {:X (quote c)}))
+
       ;; Comprehensive integration: recursion + stratified negation
       ;; + aggregation + comparison composed in a single program.
       ;; (Uses _Anything as a regular var instead of `_` so the