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/tests/builtins.sx

@@ -239,7 +239,17 @@
         "safe — = binds head var"
         (dl-bt-throws?
           (fn () (dl-program "p(a). p(b). x(Y) :- p(X), =(Y, X).")))
-        false))))
+        false)
+
+      ;; Division by zero raises with a clear error. Without this guard
+      ;; SX's `/` returned IEEE infinity, which then silently flowed
+      ;; through comparisons and aggregations.
+      (dl-bt-test!
+        "is — division by zero raises"
+        (dl-bt-throws?
+          (fn ()
+            (dl-eval "p(10). q(R) :- p(X), is(R, /(X, 0))." "?- q(R).")))
+        true))))
 
 (define
   dl-builtins-tests-run!