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/test262-scoreboard.json

@@ -1,30 +1,42 @@
 {
   "totals": {
-    "pass": 43,
-    "fail": 4,
-    "skip": 0,
-    "timeout": 3,
+    "pass": 17,
+    "fail": 21,
+    "skip": 5,
+    "timeout": 7,
     "total": 50,
-    "runnable": 50,
-    "pass_rate": 86.0
+    "runnable": 45,
+    "pass_rate": 37.8
   },
   "categories": [
     {
-      "category": "built-ins/Number",
+      "category": "built-ins/Array",
       "total": 50,
-      "pass": 43,
-      "fail": 4,
-      "skip": 0,
-      "timeout": 3,
-      "pass_rate": 86.0,
+      "pass": 17,
+      "fail": 21,
+      "skip": 5,
+      "timeout": 7,
+      "pass_rate": 37.8,
       "top_failures": [
         [
           "Test262Error (assertion failed)",
-          4
+          16
         ],
         [
           "Timeout",
-          3
+          7
+        ],
+        [
+          "TypeError: not a function",
+          2
+        ],
+        [
+          "ReferenceError (undefined symbol)",
+          2
+        ],
+        [
+          "Unhandled: Not callable: {:2 43 :1 42 :0 41 :length 3}\\",
+          1
         ]
       ]
     }
@@ -32,14 +44,26 @@
   "top_failure_modes": [
     [
       "Test262Error (assertion failed)",
-      4
+      16
     ],
     [
       "Timeout",
-      3
+      7
+    ],
+    [
+      "TypeError: not a function",
+      2
+    ],
+    [
+      "ReferenceError (undefined symbol)",
+      2
+    ],
+    [
+      "Unhandled: Not callable: {:2 43 :1 42 :0 41 :length 3}\\",
+      1
     ]
   ],
   "pinned_commit": "d5e73fc8d2c663554fb72e2380a8c2bc1a318a33",
-  "elapsed_seconds": 71.8,
+  "elapsed_seconds": 151.1,
   "workers": 1
 }