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.md

@@ -1,13 +1,13 @@
 # test262 scoreboard
 
 Pinned commit: `d5e73fc8d2c663554fb72e2380a8c2bc1a318a33`
-Wall time: 37.7s
+Wall time: 40.5s
 
-**Total:** 58/100 runnable passed (58.0%). Raw: pass=58 fail=33 skip=1130 timeout=9 total=1230.
+**Total:** 62/100 runnable passed (62.0%). Raw: pass=62 fail=29 skip=1130 timeout=9 total=1230.
 
 ## Top failure modes
 
-- **28x** Test262Error (assertion failed)
+- **24x** Test262Error (assertion failed)
 - **9x** Timeout
 - **1x** Unhandled: Not callable: \\\
 - **1x** ReferenceError (undefined symbol)
@@ -19,13 +19,13 @@ Wall time: 37.7s
 
 | Category | Pass | Fail | Skip | Timeout | Total | Pass % |
 |---|---:|---:|---:|---:|---:|---:|
-| built-ins/String | 58 | 33 | 1123 | 9 | 1223 | 58.0% |
+| built-ins/String | 62 | 29 | 1123 | 9 | 1223 | 62.0% |
 
 ## Per-category top failures (min 10 runnable, worst first)
 
-### built-ins/String  (58/100 — 58.0%)
+### built-ins/String  (62/100 — 62.0%)
 
-- **28x** Test262Error (assertion failed)
+- **24x** Test262Error (assertion failed)
 - **9x** Timeout
 - **1x** Unhandled: Not callable: \\\
 - **1x** ReferenceError (undefined symbol)