datalog: aggregate validates that agg-var appears in goal

`count(N, Y, p(X))` silently returned `N = 1` because `Y` was never
bound by the goal — every match contributed the same unbound symbol
which dl-val-member? deduped to a single entry. Similarly:

  sum(S, Y, p(X))    => raises "expected number, got symbol"
  findall(L, Y, p(X)) => L = (Y)  (a list containing the unbound symbol)
  count(N, Y, p(X))   => N = 1    (silent garbage)

Added a third validator in dl-eval-aggregate: the agg-var must
syntactically appear among the goal's variables. Error names the
variable and the goal and explains why the result would be
meaningless.

1 new test; conformance 263/263.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

lib/datalog/scoreboard.md

@@ -1,6 +1,6 @@
 # datalog scoreboard
 
-**262 / 262 passing** (0 failure(s)).
+**263 / 263 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 | 22 | 22 | ok |
+| aggregates | 23 | 23 | ok |
 | api | 22 | 22 | ok |
 | magic | 36 | 36 | ok |
 | demo | 21 | 21 | ok |