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/aggregates.sx

@@ -113,6 +113,14 @@
           (error (str "datalog aggregate (" op
                       "): third arg must be a positive literal, got "
                       goal)))
+        ((not (dl-member-string?
+                (symbol->string agg-var)
+                (dl-vars-of goal)))
+          (error (str "datalog aggregate (" op
+                      "): aggregation variable " agg-var
+                      " does not appear in the goal " goal
+                      " — without it every match contributes the same "
+                      "(unbound) value and the result is meaningless")))
         (else
           (let ((vals (list)))
             (do