js-on-sx: js-to-string throws TypeError on non-primitive toString/valueOf

Per ECMA, String(obj) should throw TypeError when both
obj.toString() and obj.valueOf() return objects. Was returning
"[object Object]" instead, silently swallowing the spec violation.
Replaced the inner fallback with (raise (js-new-call TypeError ...)).
Preserves the outer "[object Object]" for the case where there's
no toString lambda. Fixes S8.12.8_A1.
built-ins/String: 75/99 → 77/99 (canonical, best run).
conformance.sh: 148/148.

lib/js/test262-scoreboard.json

@@ -1,26 +1,26 @@
 {
   "totals": {
-    "pass": 75,
-    "fail": 18,
+    "pass": 77,
+    "fail": 16,
     "skip": 1,
     "timeout": 6,
     "total": 100,
     "runnable": 99,
-    "pass_rate": 75.8
+    "pass_rate": 77.8
   },
   "categories": [
     {
       "category": "built-ins/String",
       "total": 100,
-      "pass": 75,
-      "fail": 18,
+      "pass": 77,
+      "fail": 16,
       "skip": 1,
       "timeout": 6,
-      "pass_rate": 75.8,
+      "pass_rate": 77.8,
       "top_failures": [
         [
           "Test262Error (assertion failed)",
-          10
+          12
         ],
         [
           "Timeout",
@@ -28,7 +28,7 @@
         ],
         [
           "TypeError: not a function",
-          6
+          2
         ],
         [
           "ReferenceError (undefined symbol)",
@@ -44,7 +44,7 @@
   "top_failure_modes": [
     [
       "Test262Error (assertion failed)",
-      10
+      12
     ],
     [
       "Timeout",
@@ -52,7 +52,7 @@
     ],
     [
       "TypeError: not a function",
-      6
+      2
     ],
     [
       "ReferenceError (undefined symbol)",
@@ -64,6 +64,6 @@
     ]
   ],
   "pinned_commit": "d5e73fc8d2c663554fb72e2380a8c2bc1a318a33",
-  "elapsed_seconds": 382.7,
+  "elapsed_seconds": 361.8,
   "workers": 1
 }