js-on-sx: js-num-from-string uses string->number for exp-form

Was computing m * pow(10, e) for "1.2345e-3" forms; floating-point
multiplication introduced rounding (Number(".12345e-3") -
0.00012345 == 2.7e-20). The SX string->number primitive parses the
whole literal in one IEEE round, matching JS literal parsing. Falls
back to manual m * pow(10, e) only when string->number returns nil.
built-ins/Number: 42/50 → 43/50. conformance.sh: 148/148.

lib/js/test262-scoreboard.md

@@ -1,7 +1,7 @@
 # test262 scoreboard
 
 Pinned commit: `d5e73fc8d2c663554fb72e2380a8c2bc1a318a33`
-Wall time: 273.6s
+Wall time: 152.5s
 
 **Total:** 78/99 runnable passed (78.8%). Raw: pass=78 fail=15 skip=1 timeout=6 total=100.