js-on-sx: lower array pad bail-out to 1M to kill remaining hang

The 2^32-1 threshold still allowed indices like 2147483648 to pad
billions of undefineds. Without sparse-array support there's no
semantic value in >1M padding; lowering the bail turns those tests
into fast assertion fails instead of timeouts.
built-ins/Array timeouts: 2 → 1. conformance.sh: 148/148.

lib/js/runtime.sx

@@ -2866,7 +2866,7 @@
             ((i (js-num-to-int key)) (n (len lst)))
             (cond
               ((< i 0) nil)
-              ((>= i 4294967295) nil)
+              ((>= i 1000000) nil)
               ((< i n) (set-nth! lst i val))
               ((= i n) (append! lst val))
               (else (do (js-pad-list! lst n i) (append! lst val))))))
@@ -2877,7 +2877,7 @@
             ((target (js-num-to-int (js-to-number val))) (n (len lst)))
             (cond
               ((< target 0) nil)
-              ((>= target 4294967295) nil)
+              ((>= target 1000000) nil)
               ((> target n) (js-pad-list! lst n target))
               (else nil))))
         (else nil))))