js-on-sx: strip leading zeros from exponent in num→string (+3)

SX's (str 1e-7) gives "1e-07" but JS spec is "1e-7" — no padding, no
leading zeros in the exponent (sign stays). We stepped through:
  mant "e" expraw  →  mant "e" (sign (strip-zeros body))

Added four small helpers: js-normalize-num-str, js-split-sign,
js-strip-leading-zeros, js-strip-zeros-loop. All pure string walkers.

Unit 521/522, slice 148/148 unchanged.
String 40 → 42, Number 75 → 76 (+3 total).
Fixes S9.8.1_A9_T1, fromCharCode/S9.7_A3.1_T1..T2 family.

lib/js/runtime.sx

@@ -1195,7 +1195,52 @@
       ((js-number-is-nan n) "NaN")
       ((= n (js-infinity-value)) "Infinity")
       ((= n (- 0 (js-infinity-value))) "-Infinity")
-      (else (str n)))))
+      (else (js-normalize-num-str (str n))))))
+
+(define
+  js-normalize-num-str
+  (fn
+    (s)
+    (let
+      ((ei (js-find-exp-char s)))
+      (if
+        (< ei 0)
+        s
+        (let
+          ((mant (js-string-slice s 0 ei))
+            (expraw (js-string-slice s (+ ei 1) (len s))))
+          (str mant "e" (js-strip-leading-zeros expraw)))))))
+
+(define
+  js-strip-leading-zeros
+  (fn
+    (s)
+    (let
+      ((sign-and-body (js-split-sign s)))
+      (let
+        ((sign (nth sign-and-body 0)) (body (nth sign-and-body 1)))
+        (let
+          ((stripped (js-strip-zeros-loop body 0 (len body))))
+          (if (= stripped "") (str sign "0") (str sign stripped)))))))
+
+(define
+  js-split-sign
+  (fn
+    (s)
+    (cond
+      ((= s "") (list "" ""))
+      ((= (char-at s 0) "-") (list "-" (js-string-slice s 1 (len s))))
+      ((= (char-at s 0) "+") (list "" (js-string-slice s 1 (len s))))
+      (else (list "" s)))))
+
+(define
+  js-strip-zeros-loop
+  (fn
+    (s i n)
+    (cond
+      ((>= i n) "")
+      ((= (char-at s i) "0") (js-strip-zeros-loop s (+ i 1) n))
+      (else (js-string-slice s i n)))))
 
 (define
   js-add