js-on-sx: js-to-string emits comma-joined elements for SX lists

String([1,2,3]) was returning "(1 2 3)" (the SX (str v) fallback in
js-to-string fell through for SX lists). Replaced the fallback with
a list-typed branch that delegates to (js-list-join v ","). Fixes
String(arr), "" + arr, and any implicit array-to-string coercion.
built-ins/String: 65/99 → 67/99. conformance.sh: 148/148.

lib/js/test262-scoreboard.json

@@ -1,34 +1,30 @@
 {
   "totals": {
-    "pass": 65,
-    "fail": 28,
+    "pass": 67,
+    "fail": 19,
     "skip": 1,
-    "timeout": 6,
+    "timeout": 13,
     "total": 100,
     "runnable": 99,
-    "pass_rate": 65.7
+    "pass_rate": 67.7
   },
   "categories": [
     {
       "category": "built-ins/String",
       "total": 100,
-      "pass": 65,
-      "fail": 28,
+      "pass": 67,
+      "fail": 19,
       "skip": 1,
-      "timeout": 6,
-      "pass_rate": 65.7,
+      "timeout": 13,
+      "pass_rate": 67.7,
       "top_failures": [
         [
           "Test262Error (assertion failed)",
-          16
-        ],
-        [
-          "TypeError: not a function",
-          8
+          15
         ],
         [
           "Timeout",
-          6
+          13
         ],
         [
           "Unhandled: Not callable: \\\\\\",
@@ -37,6 +33,10 @@
         [
           "ReferenceError (undefined symbol)",
           1
+        ],
+        [
+          "SyntaxError (parse/unsupported syntax)",
+          1
         ]
       ]
     }
@@ -44,15 +44,11 @@
   "top_failure_modes": [
     [
       "Test262Error (assertion failed)",
-      16
-    ],
-    [
-      "TypeError: not a function",
-      8
+      15
     ],
     [
       "Timeout",
-      6
+      13
     ],
     [
       "Unhandled: Not callable: \\\\\\",
@@ -68,6 +64,6 @@
     ]
   ],
   "pinned_commit": "d5e73fc8d2c663554fb72e2380a8c2bc1a318a33",
-  "elapsed_seconds": 353.1,
+  "elapsed_seconds": 580.2,
   "workers": 1
 }