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.md

@@ -1,6 +1,6 @@
 # datalog scoreboard
 
-**257 / 257 passing** (0 failure(s)).
+**259 / 259 passing** (0 failure(s)).
 
 | Suite | Passed | Total | Status |
 |-------|--------|-------|--------|
@@ -11,7 +11,7 @@
 | builtins | 23 | 23 | ok |
 | semi_naive | 8 | 8 | ok |
 | negation | 10 | 10 | ok |
-| aggregates | 20 | 20 | ok |
+| aggregates | 22 | 22 | ok |
 | api | 20 | 20 | ok |
 | magic | 36 | 36 | ok |
 | demo | 21 | 21 | ok |