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.json

@@ -1,13 +1,13 @@
 {
   "lang": "datalog",
-  "total_passed": 251,
+  "total_passed": 253,
   "total_failed": 0,
-  "total": 251,
+  "total": 253,
   "suites": [
     {"name":"tokenize","passed":29,"failed":0,"total":29},
     {"name":"parse","passed":22,"failed":0,"total":22},
     {"name":"unify","passed":28,"failed":0,"total":28},
-    {"name":"eval","passed":36,"failed":0,"total":36},
+    {"name":"eval","passed":38,"failed":0,"total":38},
     {"name":"builtins","passed":23,"failed":0,"total":23},
     {"name":"semi_naive","passed":8,"failed":0,"total":8},
     {"name":"negation","passed":10,"failed":0,"total":10},
@@ -16,5 +16,5 @@
     {"name":"magic","passed":34,"failed":0,"total":34},
     {"name":"demo","passed":21,"failed":0,"total":21}
   ],
-  "generated": "2026-05-10T21:05:12+00:00"
+  "generated": "2026-05-10T21:09:18+00:00"
 }