js-on-sx: js-div coerces divisor to inexact

(js-div 1 0) with rational integer literals throws "rational: division
by zero" instead of producing Infinity. Wrapped the divisor in
(exact->inexact ...) so integer-by-zero now returns inf/-inf/nan
matching JS semantics. Hit by the harness's _isSameValue +0/-0 check
which calls (js-div 1 a) on JS literal arguments.
built-ins/Number: 37/50 → 41/50. built-ins/String: 77/99.
conformance.sh: 148/148.

lib/js/runtime.sx

@@ -1436,7 +1436,9 @@
 
 (define js-mul (fn (a b) (* (js-to-number a) (js-to-number b))))
 
-(define js-div (fn (a b) (/ (js-to-number a) (js-to-number b))))
+(define
+  js-div
+  (fn (a b) (/ (js-to-number a) (exact->inexact (js-to-number b)))))
 
 (define js-mod (fn (a b) (mod (js-to-number a) (js-to-number b))))