datalog: arith / by zero raises instead of returning inf

`is(R, /(X, 0))` was silently producing IEEE infinity:

  (dl-eval "p(10). q(R) :- p(X), is(R, /(X, 0))." "?- q(R).")
  => ({:R inf})

That value then flowed through comparisons (anything < inf, anything
> inf) and aggregations (sum of inf, max of inf) producing nonsense
results downstream. `dl-eval-arith` now checks the divisor before
the host `/` and raises "division by zero in <expr>" — surfacing
the bug at its source rather than letting infinity propagate.

1 new test; conformance 264/264.

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

lib/datalog/builtins.sx

@@ -72,7 +72,13 @@
                     ((= rel "+") (+ a b))
                     ((= rel "-") (- a b))
                     ((= rel "*") (* a b))
-                    ((= rel "/") (/ a b))
+                    ((= rel "/")
+                      (cond
+                        ((= b 0)
+                          (error
+                            (str "datalog arith: division by zero in "
+                                 w)))
+                        (else (/ a b))))
                     (else (error (str "datalog arith: unknown op " rel)))))))))
         (else (error (str "datalog arith: not a number — " w)))))))