js-on-sx: high-precision number-to-string via round-trip + digit extraction

- js-big-int-str-loop: extract decimal digits from integer-valued float
- js-find-decimal-k: find min decimal places k where round(n*10^k)/10^k == n
- js-format-decimal-digits: insert decimal point into digit string at position (len-k)
- js-number-to-string: if 6-sig-fig round-trip fails AND n in [1e-6, 1e21),
  use digit extraction for full precision (up to 17 sig figs)
- String(1.0000001)="1.0000001", String(1/3)="0.3333333333333333"
- String test262 subset: 58→62/100

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/js/test262-scoreboard.json

@@ -1,26 +1,26 @@
 {
   "totals": {
-    "pass": 58,
-    "fail": 33,
+    "pass": 62,
+    "fail": 29,
     "skip": 1130,
     "timeout": 9,
     "total": 1230,
     "runnable": 100,
-    "pass_rate": 58.0
+    "pass_rate": 62.0
   },
   "categories": [
     {
       "category": "built-ins/String",
       "total": 1223,
-      "pass": 58,
-      "fail": 33,
+      "pass": 62,
+      "fail": 29,
       "skip": 1123,
       "timeout": 9,
-      "pass_rate": 58.0,
+      "pass_rate": 62.0,
       "top_failures": [
         [
           "Test262Error (assertion failed)",
-          28
+          24
         ],
         [
           "Timeout",
@@ -54,7 +54,7 @@
   "top_failure_modes": [
     [
       "Test262Error (assertion failed)",
-      28
+      24
     ],
     [
       "Timeout",
@@ -82,6 +82,6 @@
     ]
   ],
   "pinned_commit": "d5e73fc8d2c663554fb72e2380a8c2bc1a318a33",
-  "elapsed_seconds": 37.7,
+  "elapsed_seconds": 40.5,
   "workers": 7
 }