js-on-sx: scope var defines + js-args for call args

JS top-level var was emitting (define <name> X) at SX top level,
permanently rebinding any SX primitive of that name (e.g. var list
= X broke (list ...) globally). Two-part fix:
1. wrap transpiled program in (let () ...) in js-eval so defines
   scope to the eval and don't leak.
2. rename call-args constructor in js-transpile-args from list to
   js-args (a variadic alias) so even within the eval's own scope,
   JS vars named list don't shadow arg construction.
Array-literal transpile keeps list (arrays must be mutable).
built-ins/Object: 41/50 → 42/50. conformance.sh: 148/148.

lib/js/test262-scoreboard.json

@@ -1,61 +1,61 @@
 {
   "totals": {
-    "pass": 41,
-    "fail": 9,
-    "skip": 0,
-    "timeout": 0,
-    "total": 50,
-    "runnable": 50,
-    "pass_rate": 82.0
+    "pass": 77,
+    "fail": 15,
+    "skip": 1,
+    "timeout": 7,
+    "total": 100,
+    "runnable": 99,
+    "pass_rate": 77.8
   },
   "categories": [
     {
-      "category": "built-ins/Object",
-      "total": 50,
-      "pass": 41,
-      "fail": 9,
-      "skip": 0,
-      "timeout": 0,
-      "pass_rate": 82.0,
+      "category": "built-ins/String",
+      "total": 100,
+      "pass": 77,
+      "fail": 15,
+      "skip": 1,
+      "timeout": 7,
+      "pass_rate": 77.8,
       "top_failures": [
+        [
+          "Test262Error (assertion failed)",
+          13
+        ],
+        [
+          "Timeout",
+          7
+        ],
         [
           "ReferenceError (undefined symbol)",
-          4
+          1
         ],
         [
           "SyntaxError (parse/unsupported syntax)",
-          2
-        ],
-        [
-          "Test262Error (assertion failed)",
-          2
-        ],
-        [
-          "runner-error: sx_server closed stdout mid-epoch",
           1
         ]
       ]
     }
   ],
   "top_failure_modes": [
+    [
+      "Test262Error (assertion failed)",
+      13
+    ],
+    [
+      "Timeout",
+      7
+    ],
     [
       "ReferenceError (undefined symbol)",
-      4
+      1
     ],
     [
       "SyntaxError (parse/unsupported syntax)",
-      2
-    ],
-    [
-      "Test262Error (assertion failed)",
-      2
-    ],
-    [
-      "runner-error: sx_server closed stdout mid-epoch",
       1
     ]
   ],
   "pinned_commit": "d5e73fc8d2c663554fb72e2380a8c2bc1a318a33",
-  "elapsed_seconds": 37.1,
+  "elapsed_seconds": 486.8,
   "workers": 1
 }