datalog: dl-magic-query falls back on built-in/agg/neg goals (222/222)

Bug: dl-magic-query was always trying to seed a magic_<rel>^<adn>
fact for the query goal. For aggregate goals like (count N X (p X))
this produced a non-ground "fact" (magic_count^... N X (p X)) and
dl-add-fact! correctly rejected it, surfacing as an error.

Fix: dl-magic-query now detects built-in / aggregate / negation
goals up front and dispatches to plain dl-query for those cases —
magic-sets only applies to positive non-builtin literals against
rule-defined relations. Other shapes don't benefit from the
rewrite anyway.

1 new test confirms (count N X (p X)) returns the expected
{:N 3} via dl-magic-query.

lib/datalog/tests/magic.sx

@@ -197,6 +197,18 @@
         6)
 
       ;; dl-magic-query: end-to-end driver, doesn't mutate caller's db.
+      ;; dl-magic-query falls back to dl-query for built-in,
+      ;; aggregate, and negation goals (the magic seed would
+      ;; otherwise be non-ground).
+      (dl-mt-test! "magic-query falls back on aggregate"
+        (let
+          ((r (dl-magic-query
+                (dl-program "p(1). p(2). p(3).")
+                (list (quote count) (quote N) (quote X)
+                      (list (quote p) (quote X))))))
+          (and (= (len r) 1) (= (get (first r) "N") 3)))
+        true)
+
       (dl-mt-test! "magic-query equivalent to dl-query"
         (let
           ((db (dl-program