datalog: reject malformed dict body literals

A dict in a rule body that isn't `{:neg <positive-lit>}` (the only
recognised dict shape) used to silently fall through every dispatch
clause in dl-rule-check-safety, contributing zero bound variables.
The user would then see a confusing "head variable(s) X do not
appear in any positive body literal" pointing at the head — not at
the actual bug in the body. Typos like `{:negs ...}` are the typical
trigger.

dl-process-lit! now flags both:

  - a dict that lacks :neg
  - a bare number / string / symbol used as a body lit

with a clear error naming the offending literal.

1 new regression test; conformance 265/265.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

lib/datalog/tests/eval.sx

@@ -221,6 +221,19 @@
               "banned(a). u(a). vip(X) :- u(X), not(not(banned(X))).")))
         true)
 
+      ;; A dict body literal that isn't `{:neg ...}` is almost always a
+      ;; typo — it would otherwise silently fall through to a confusing
+      ;; head-var-unbound safety error. Now caught with a clear message.
+      (dl-et-test!
+        "dict body lit without :neg rejected"
+        (dl-et-throws?
+          (fn ()
+            (let ((db (dl-make-db)))
+              (dl-add-rule! db
+                {:head (list (quote p) (quote X))
+                 :body (list {:weird "stuff"})}))))
+        true)
+
       (dl-et-test!
         "unsafe head var"
         (dl-et-throws? (fn () (dl-program "p(X, Y) :- q(X).")))