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/builtins.sx

@@ -301,6 +301,18 @@
               (cond
                 ((and (dict? lit) (has-key? lit :neg))
                   (dl-process-neg! lit))
+                ;; A bare dict that is not a recognised negation is
+                ;; almost certainly a typo (e.g. `{:negs ...}` instead
+                ;; of `{:neg ...}`). Without this guard the dict would
+                ;; silently fall through every clause; the head safety
+                ;; check would then flag the head variables as unbound
+                ;; even though the real bug is the malformed body lit.
+                ((dict? lit)
+                  (set! err
+                    (str "body literal is a dict but lacks :neg — "
+                         "the only dict-shaped body lit recognised is "
+                         "{:neg <positive-lit>} for stratified "
+                         "negation, got " lit)))
                 ((dl-aggregate? lit) (dl-process-agg! lit))
                 ((dl-eq? lit) (dl-process-eq! lit))
                 ((dl-is? lit) (dl-process-is! lit))
@@ -315,7 +327,15 @@
                                "syntax; nested `not(...)` is not supported "
                                "(use stratified negation via an "
                                "intermediate relation)")))
-                      (else (dl-add-bound! (dl-vars-of lit))))))))))
+                      (else (dl-add-bound! (dl-vars-of lit))))))
+                (else
+                  ;; Anything that's not a dict, not a list, or an
+                  ;; empty list. Numbers / strings / symbols as body
+                  ;; lits don't make sense — surface the type.
+                  (set! err
+                    (str "body literal must be a positive lit, "
+                         "built-in, aggregate, or {:neg ...} dict, "
+                         "got " lit)))))))
         (for-each dl-process-lit! body)
         (when
           (nil? err)