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/runtime.sx

@@ -1194,6 +1194,53 @@
         (+ i 1)
         (str acc (js-to-string (nth parts i)))))))
 
+(define
+  js-big-int-str-loop
+  (fn
+    (n acc)
+    (if
+      (< n 1)
+      (if (= acc "") "0" acc)
+      (let
+        ((d (floor (- n (* 10 (floor (/ n 10)))))))
+        (js-big-int-str-loop
+          (floor (/ n 10))
+          (str (js-string-slice "0123456789" d (+ d 1)) acc))))))
+
+(define
+  js-find-decimal-k
+  (fn
+    (n k)
+    (if
+      (> k 17)
+      17
+      (let
+        ((big-int (round (* n (js-pow-int 10 k)))))
+        (if
+          (= (/ big-int (js-pow-int 10 k)) n)
+          k
+          (js-find-decimal-k n (+ k 1)))))))
+
+(define
+  js-format-decimal-digits
+  (fn
+    (digits k)
+    (if
+      (= k 0)
+      digits
+      (let
+        ((dlen (len digits)))
+        (if
+          (> dlen k)
+          (str
+            (js-string-slice digits 0 (- dlen k))
+            "."
+            (js-string-slice digits (- dlen k) dlen))
+          (if
+            (= dlen k)
+            (str "0." digits)
+            (str "0." (js-string-repeat "0" (- k dlen)) digits)))))))
+
 (define
   js-expand-sci-notation
   (fn
@@ -1234,9 +1281,9 @@
           (let
             ((s0 (js-normalize-num-str (str pos-n))))
             (let
-              ((ei (js-string-index-of s0 "e" 0)))
+              ((n2 (js-to-number s0)))
               (let
-                ((precise (if (< ei 0) s0 (let ((exp-n (js-to-number (js-string-slice s0 (+ ei 1) (len s0))))) (if (and (>= exp-n -6) (<= exp-n 20)) (js-expand-sci-notation (js-string-slice s0 0 ei) exp-n) (if (>= exp-n 0) (str (js-string-slice s0 0 (+ ei 1)) "+" (str exp-n)) s0))))))
+                ((precise (if (= n2 pos-n) (let ((ei (js-string-index-of s0 "e" 0))) (if (< ei 0) s0 (let ((exp-n (js-to-number (js-string-slice s0 (+ ei 1) (len s0))))) (if (and (>= exp-n -6) (<= exp-n 20)) (js-expand-sci-notation (js-string-slice s0 0 ei) exp-n) (if (>= exp-n 0) (str (js-string-slice s0 0 (+ ei 1)) "+" (str exp-n)) s0))))) (if (and (>= pos-n 1e-06) (< pos-n 1e+21)) (let ((k (js-find-decimal-k pos-n 0))) (let ((big-int (round (* pos-n (js-pow-int 10 k))))) (js-format-decimal-digits (js-big-int-str-loop big-int "") k))) (let ((ei (js-string-index-of s0 "e" 0))) (if (< ei 0) s0 (let ((exp-n (js-to-number (js-string-slice s0 (+ ei 1) (len s0))))) (if (>= exp-n 0) (str (js-string-slice s0 0 (+ ei 1)) "+" (str exp-n)) s0))))))))
                 (if (< n 0) (str "-" precise) precise)))))))))
 
 (define