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/scoreboard.json

@@ -1,13 +1,13 @@
 {
   "lang": "datalog",
-  "total_passed": 264,
+  "total_passed": 265,
   "total_failed": 0,
-  "total": 264,
+  "total": 265,
   "suites": [
     {"name":"tokenize","passed":30,"failed":0,"total":30},
     {"name":"parse","passed":22,"failed":0,"total":22},
     {"name":"unify","passed":29,"failed":0,"total":29},
-    {"name":"eval","passed":39,"failed":0,"total":39},
+    {"name":"eval","passed":40,"failed":0,"total":40},
     {"name":"builtins","passed":24,"failed":0,"total":24},
     {"name":"semi_naive","passed":8,"failed":0,"total":8},
     {"name":"negation","passed":10,"failed":0,"total":10},
@@ -16,5 +16,5 @@
     {"name":"magic","passed":36,"failed":0,"total":36},
     {"name":"demo","passed":21,"failed":0,"total":21}
   ],
-  "generated": "2026-05-11T07:59:10+00:00"
+  "generated": "2026-05-11T08:03:45+00:00"
 }