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/magic.sx

@@ -382,6 +382,11 @@
     ;; (built-ins, aggregates, EDB-only relations) the seed is either
     ;; non-ground or unused; fall back to dl-query.
     (cond
+      ((not (and (list? query-goal)
+                 (> (len query-goal) 0)
+                 (symbol? (first query-goal))))
+        (error (str "dl-magic-query: goal must be a positive literal "
+                    "(non-empty list with a symbol head), got " query-goal)))
       ((or (dl-builtin? query-goal)
            (dl-aggregate? query-goal)
            (and (dict? query-goal) (has-key? query-goal :neg)))