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)))

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"
 }

lib/datalog/scoreboard.md

@@ -1,6 +1,6 @@
 # datalog scoreboard
 
-**253 / 253 passing** (0 failure(s)).
+**255 / 255 passing** (0 failure(s)).
 
 | Suite | Passed | Total | Status |
 |-------|--------|-------|--------|
@@ -13,5 +13,5 @@
 | negation | 10 | 10 | ok |
 | aggregates | 20 | 20 | ok |
 | api | 20 | 20 | ok |
-| magic | 34 | 34 | ok |
+| magic | 36 | 36 | ok |
 | demo | 21 | 21 | ok |

lib/datalog/tests/magic.sx

@@ -242,6 +242,24 @@
             (= (len semi) (len magic))))
         true)
 
+      ;; Shape validation: dl-magic-query rejects non-list / non-
+      ;; dict goal shapes cleanly rather than crashing in `rest`.
+      (dl-mt-test! "magic rejects string goal"
+        (let ((threw false))
+          (do
+            (guard (e (#t (set! threw true)))
+              (dl-magic-query (dl-make-db) "foo"))
+            threw))
+        true)
+
+      (dl-mt-test! "magic rejects bare dict goal"
+        (let ((threw false))
+          (do
+            (guard (e (#t (set! threw true)))
+              (dl-magic-query (dl-make-db) {:foo "bar"}))
+            threw))
+        true)
+
       ;; 3-stratum program under magic — distinct rule heads at
       ;; strata 0/1/2 must all rewrite via the worklist.
       (dl-mt-test! "magic 3-stratum program"