datalog: aggregate arg validators (259/259)

Bug: dl-eval-aggregate accepted non-variable agg-vars and non-
literal goals silently, producing weird/incorrect counts:
- `count(N, 5, p(X))` would compute count over the single
  constant 5 (always 1), ignoring p entirely.
- `count(N, X, 42)` would crash with "unknown body-literal
  shape" at saturation time rather than at rule-add time.

Fix: dl-eval-aggregate now validates up front that the second
arg is a variable (the value to aggregate) and the third arg is
a positive literal (the goal). Errors are descriptive and
include the offending argument.

2 new aggregate tests.

lib/datalog/scoreboard.json

@@ -1,8 +1,8 @@
 {
   "lang": "datalog",
-  "total_passed": 257,
+  "total_passed": 259,
   "total_failed": 0,
-  "total": 257,
+  "total": 259,
   "suites": [
     {"name":"tokenize","passed":30,"failed":0,"total":30},
     {"name":"parse","passed":22,"failed":0,"total":22},
@@ -11,10 +11,10 @@
     {"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},
-    {"name":"aggregates","passed":20,"failed":0,"total":20},
+    {"name":"aggregates","passed":22,"failed":0,"total":22},
     {"name":"api","passed":20,"failed":0,"total":20},
     {"name":"magic","passed":36,"failed":0,"total":36},
     {"name":"demo","passed":21,"failed":0,"total":21}
   ],
-  "generated": "2026-05-11T07:20:07+00:00"
+  "generated": "2026-05-11T07:26:33+00:00"
 }