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/runtime.sx

@@ -1123,23 +1123,28 @@
             (if
               (>= esplit 0)
               (let
-                ((mant (js-string-slice trimmed 0 esplit))
-                  (expstr
-                    (js-string-slice
-                      trimmed
-                      (+ esplit 1)
-                      (len trimmed))))
-                (let
-                  ((m (js-parse-decimal mant 0 0 1 false 0))
-                    (e
-                      (js-parse-decimal
-                        expstr
-                        0
-                        0
-                        1
-                        false
-                        0)))
-                  (* m (pow 10 e))))
+                ((parsed (string->number trimmed)))
+                (if
+                  (= parsed nil)
+                  (let
+                    ((mant (js-string-slice trimmed 0 esplit))
+                      (expstr
+                        (js-string-slice
+                          trimmed
+                          (+ esplit 1)
+                          (len trimmed))))
+                    (let
+                      ((m (js-parse-decimal mant 0 0 1 false 0))
+                        (e
+                          (js-parse-decimal
+                            expstr
+                            0
+                            0
+                            1
+                            false
+                            0)))
+                      (* m (pow 10 e))))
+                  parsed))
               (js-parse-decimal
                 trimmed
                 0

lib/js/test262-scoreboard.json

@@ -56,6 +56,6 @@
     ]
   ],
   "pinned_commit": "d5e73fc8d2c663554fb72e2380a8c2bc1a318a33",
-  "elapsed_seconds": 273.6,
+  "elapsed_seconds": 152.5,
   "workers": 1
 }

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.