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/magic.sx

@@ -376,16 +376,26 @@
   dl-magic-query
   (fn
     (db query-goal)
-    (let
-      ((query-rel (dl-rel-name query-goal))
-        (query-adn (dl-adorn-goal query-goal)))
-      (let
-        ((query-args (dl-bound-args query-goal query-adn))
-          (rules (dl-rules db)))
+    ;; Magic-sets only applies to positive non-builtin / non-aggregate
+    ;; literals against rule-defined relations. For other goal shapes
+    ;; (built-ins, aggregates, EDB-only relations) the seed is either
+    ;; non-ground or unused; fall back to dl-query.
+    (cond
+      ((or (dl-builtin? query-goal)
+           (dl-aggregate? query-goal)
+           (and (dict? query-goal) (has-key? query-goal :neg)))
+        (dl-query db query-goal))
+      (else
         (let
-          ((rewritten (dl-magic-rewrite rules query-rel query-adn query-args))
-            (mdb (dl-make-db))
-            (rule-heads (dl-magic-rule-heads rules)))
+          ((query-rel (dl-rel-name query-goal))
+            (query-adn (dl-adorn-goal query-goal)))
+          (let
+            ((query-args (dl-bound-args query-goal query-adn))
+              (rules (dl-rules db)))
+            (let
+              ((rewritten (dl-magic-rewrite rules query-rel query-adn query-args))
+                (mdb (dl-make-db))
+                (rule-heads (dl-magic-rule-heads rules)))
           (do
             ;; Copy EDB facts (relations not headed by any caller rule).
             (for-each
@@ -400,4 +410,4 @@
             ;; Seed + rewritten rules.
             (dl-add-fact! mdb (get rewritten :seed))
             (for-each (fn (r) (dl-add-rule! mdb r)) (get rewritten :rules))
-            (dl-query mdb query-goal)))))))
+            (dl-query mdb query-goal)))))))))