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/scoreboard.json

@@ -1,8 +1,8 @@
 {
   "lang": "datalog",
-  "total_passed": 221,
+  "total_passed": 222,
   "total_failed": 0,
-  "total": 221,
+  "total": 222,
   "suites": [
     {"name":"tokenize","passed":26,"failed":0,"total":26},
     {"name":"parse","passed":18,"failed":0,"total":18},
@@ -13,8 +13,8 @@
     {"name":"negation","passed":10,"failed":0,"total":10},
     {"name":"aggregates","passed":19,"failed":0,"total":19},
     {"name":"api","passed":20,"failed":0,"total":20},
-    {"name":"magic","passed":23,"failed":0,"total":23},
+    {"name":"magic","passed":24,"failed":0,"total":24},
     {"name":"demo","passed":21,"failed":0,"total":21}
   ],
-  "generated": "2026-05-08T10:29:04+00:00"
+  "generated": "2026-05-08T10:31:43+00:00"
 }