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/db.sx

@@ -318,6 +318,15 @@
         (error (str "dl-add-rule!: expected rule dict, got " rule)))
       ((not (has-key? rule :head))
         (error (str "dl-add-rule!: rule missing :head, got " rule)))
+      ((not (and (list? (get rule :head))
+                 (> (len (get rule :head)) 0)
+                 (symbol? (first (get rule :head)))))
+        (error (str "dl-add-rule!: head must be a non-empty list "
+                    "starting with a relation-name symbol, got "
+                    (get rule :head))))
+      ((not (list? (if (has-key? rule :body) (get rule :body) (list))))
+        (error (str "dl-add-rule!: body must be a list of literals, got "
+                    (get rule :body))))
       ((dl-reserved-rel? (dl-rel-name (get rule :head)))
         (error (str "dl-add-rule!: '" (dl-rel-name (get rule :head))
                     "' is a reserved name (built-in / aggregate / negation)")))