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/test262-scoreboard.json

@@ -20,16 +20,12 @@
       "top_failures": [
         [
           "Test262Error (assertion failed)",
-          12
+          14
         ],
         [
           "Timeout",
           6
         ],
-        [
-          "TypeError: not a function",
-          2
-        ],
         [
           "ReferenceError (undefined symbol)",
           1
@@ -44,16 +40,12 @@
   "top_failure_modes": [
     [
       "Test262Error (assertion failed)",
-      12
+      14
     ],
     [
       "Timeout",
       6
     ],
-    [
-      "TypeError: not a function",
-      2
-    ],
     [
       "ReferenceError (undefined symbol)",
       1
@@ -64,6 +56,6 @@
     ]
   ],
   "pinned_commit": "d5e73fc8d2c663554fb72e2380a8c2bc1a318a33",
-  "elapsed_seconds": 361.8,
+  "elapsed_seconds": 420.7,
   "workers": 1
 }