js-on-sx: js-num-to-int guards NaN/Infinity → 0 (+2 String)

Spec ToUint16 (String.fromCharCode argument coercion) maps non-finite
values to 0. We had bare (floor v) which left inf/-inf/nan through,
breaking:
  String.fromCharCode(Infinity).charCodeAt(0) === 0      // was "" → err
  String.fromCharCode(NaN).charCodeAt(0) === 0           // was "" → err

Add NaN/inf/-inf guards returning 0 before the floor+signed-flip path.

Unit 521/522, slice 148/148 unchanged.
String 38/100 → 40/100 (+2: fromCharCode/S9.7_A1, S9.7_A2.1).

lib/js/runtime.sx

@@ -2291,8 +2291,13 @@
   (fn
     (v)
     (let
-      ((n (if (number? v) v (js-to-number v))))
-      (if (>= n 0) (floor n) (- 0 (floor (- 0 n)))))))
+      ((n (js-to-number v)))
+      (cond
+        ((js-number-is-nan n) 0)
+        ((= n (js-infinity-value)) 0)
+        ((= n (- 0 (js-infinity-value))) 0)
+        ((>= n 0) (floor n))
+        (else (- 0 (floor (- 0 n))))))))
 
 (define dict-has? (fn (d k) (contains? (keys d) k)))