js-on-sx: js-num-from-string uses pow (float) for exponent

js-pow-int 10 20 overflows int64 (10^20 > 2^63), so numeric literals
like 1e20 and 100000000000000000000 were parsing as
-1457092405402533888. The pow primitive uses float-domain
exponentiation and produces 1e+20 correctly. Single call swap in
js-num-from-string. built-ins/String (with --restart-every 1):
67/99 → 70/99. conformance.sh: 148/148.

lib/js/test262-scoreboard.md

@@ -1,14 +1,15 @@
 # test262 scoreboard
 
 Pinned commit: `d5e73fc8d2c663554fb72e2380a8c2bc1a318a33`
-Wall time: 580.2s
+Wall time: 364.7s
 
-**Total:** 67/99 runnable passed (67.7%). Raw: pass=67 fail=19 skip=1 timeout=13 total=100.
+**Total:** 67/99 runnable passed (67.7%). Raw: pass=67 fail=26 skip=1 timeout=6 total=100.
 
 ## Top failure modes
 
-- **15x** Test262Error (assertion failed)
-- **13x** Timeout
+- **14x** Test262Error (assertion failed)
+- **8x** TypeError: not a function
+- **6x** Timeout
 - **2x** Unhandled: Not callable: \\\
 - **1x** ReferenceError (undefined symbol)
 - **1x** SyntaxError (parse/unsupported syntax)
@@ -17,14 +18,14 @@ Wall time: 580.2s
 
 | Category | Pass | Fail | Skip | Timeout | Total | Pass % |
 |---|---:|---:|---:|---:|---:|---:|
-| built-ins/String | 67 | 19 | 1 | 13 | 100 | 67.7% |
+| built-ins/String | 67 | 26 | 1 | 6 | 100 | 67.7% |
 
 ## Per-category top failures (min 10 runnable, worst first)
 
 ### built-ins/String  (67/99 — 67.7%)
 
-- **15x** Test262Error (assertion failed)
-- **13x** Timeout
+- **14x** Test262Error (assertion failed)
+- **8x** TypeError: not a function
+- **6x** Timeout
 - **2x** Unhandled: Not callable: \\\
 - **1x** ReferenceError (undefined symbol)
-- **1x** SyntaxError (parse/unsupported syntax)