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.md

@@ -1,13 +1,13 @@
 # datalog scoreboard
 
-**264 / 264 passing** (0 failure(s)).
+**265 / 265 passing** (0 failure(s)).
 
 | Suite | Passed | Total | Status |
 |-------|--------|-------|--------|
 | tokenize | 30 | 30 | ok |
 | parse | 22 | 22 | ok |
 | unify | 29 | 29 | ok |
-| eval | 39 | 39 | ok |
+| eval | 40 | 40 | ok |
 | builtins | 24 | 24 | ok |
 | semi_naive | 8 | 8 | ok |
 | negation | 10 | 10 | ok |