datalog: dl-magic-query shape validator (255/255)

Bug: dl-magic-query crashed with cryptic "rest: 1 list arg" when
the goal argument was a string, number, or arbitrary dict. The
first thing the function does is dl-rel-name + dl-adorn-goal,
both of which assume a positive-literal list shape.

Fix: explicit shape check up front. A goal must be a non-empty
list whose first element is a symbol. Otherwise raise with a
clear diagnostic. Built-in / aggregate / negation dispatch (the
fall-back to dl-query) is unchanged.

2 new magic tests cover string and bare-dict goal rejection.

lib/datalog/scoreboard.json

@@ -1,8 +1,8 @@
 {
   "lang": "datalog",
-  "total_passed": 253,
+  "total_passed": 255,
   "total_failed": 0,
-  "total": 253,
+  "total": 255,
   "suites": [
     {"name":"tokenize","passed":29,"failed":0,"total":29},
     {"name":"parse","passed":22,"failed":0,"total":22},
@@ -13,8 +13,8 @@
     {"name":"negation","passed":10,"failed":0,"total":10},
     {"name":"aggregates","passed":20,"failed":0,"total":20},
     {"name":"api","passed":20,"failed":0,"total":20},
-    {"name":"magic","passed":34,"failed":0,"total":34},
+    {"name":"magic","passed":36,"failed":0,"total":36},
     {"name":"demo","passed":21,"failed":0,"total":21}
   ],
-  "generated": "2026-05-10T21:09:18+00:00"
+  "generated": "2026-05-10T21:13:14+00:00"
 }