datalog: rule-shape validators in dl-add-rule! (253/253)

Two malformed-rule paths used to slip through:

- Empty head list `{:head () :body ()}` was accepted; the rule
  would never fire but the relation-name lookup later returned
  nil with confusing downstream errors.
- Non-list body (`{:head (...) :body 42}`) crashed in `rest`
  during safety check with a cryptic "rest: 1 list arg".

dl-add-rule! now checks head shape (non-empty list with symbol
head) and body type (list) before any safety walk. Errors are
descriptive and surface at add time rather than during the next
saturation.

2 new eval tests.

lib/datalog/scoreboard.md

@@ -1,13 +1,13 @@
 # datalog scoreboard
 
-**251 / 251 passing** (0 failure(s)).
+**253 / 253 passing** (0 failure(s)).
 
 | Suite | Passed | Total | Status |
 |-------|--------|-------|--------|
 | tokenize | 29 | 29 | ok |
 | parse | 22 | 22 | ok |
 | unify | 28 | 28 | ok |
-| eval | 36 | 36 | ok |
+| eval | 38 | 38 | ok |
 | builtins | 23 | 23 | ok |
 | semi_naive | 8 | 8 | ok |
 | negation | 10 | 10 | ok |