js-on-sx: arr.length = N extends the array

js-list-set! was a no-op for the length key. Added a clause that
pads with js-undefined via js-pad-list! when target > current.
Truncation skipped: the pop-last! SX primitive doesn't actually
mutate the list (length unchanged after the call), so no clean
way to shrink in place from SX. Extension covers common cases.
built-ins/Array: 16/45 → 17/45. conformance.sh: 148/148.

lib/js/runtime.sx

@@ -2800,7 +2800,13 @@
               (else (do (js-pad-list! lst n i) (append! lst val))))))
         ((and (= (type-of key) "string") (js-is-numeric-string? key))
           (js-list-set! lst (js-string-to-number key) val))
-        ((= key "length") nil)
+        ((= key "length")
+          (let
+            ((target (js-num-to-int (js-to-number val))) (n (len lst)))
+            (cond
+              ((< target 0) nil)
+              ((> target n) (js-pad-list! lst n target))
+              (else nil))))
         (else nil))))
   (define
     js-pad-list!