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.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 |