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

lib/js/test262-scoreboard.json

@@ -1,26 +1,26 @@
 {
   "totals": {
-    "pass": 58,
-    "fail": 33,
+    "pass": 62,
+    "fail": 29,
     "skip": 1130,
     "timeout": 9,
     "total": 1230,
     "runnable": 100,
-    "pass_rate": 58.0
+    "pass_rate": 62.0
   },
   "categories": [
     {
       "category": "built-ins/String",
       "total": 1223,
-      "pass": 58,
-      "fail": 33,
+      "pass": 62,
+      "fail": 29,
       "skip": 1123,
       "timeout": 9,
-      "pass_rate": 58.0,
+      "pass_rate": 62.0,
       "top_failures": [
         [
           "Test262Error (assertion failed)",
-          28
+          24
         ],
         [
           "Timeout",
@@ -54,7 +54,7 @@
   "top_failure_modes": [
     [
       "Test262Error (assertion failed)",
-      28
+      24
     ],
     [
       "Timeout",
@@ -82,6 +82,6 @@
     ]
   ],
   "pinned_commit": "d5e73fc8d2c663554fb72e2380a8c2bc1a318a33",
-  "elapsed_seconds": 37.7,
+  "elapsed_seconds": 40.5,
   "workers": 7
 }

lib/js/test262-scoreboard.md

@@ -1,13 +1,13 @@
 # test262 scoreboard
 
 Pinned commit: `d5e73fc8d2c663554fb72e2380a8c2bc1a318a33`
-Wall time: 37.7s
+Wall time: 40.5s
 
-**Total:** 58/100 runnable passed (58.0%). Raw: pass=58 fail=33 skip=1130 timeout=9 total=1230.
+**Total:** 62/100 runnable passed (62.0%). Raw: pass=62 fail=29 skip=1130 timeout=9 total=1230.
 
 ## Top failure modes
 
-- **28x** Test262Error (assertion failed)
+- **24x** Test262Error (assertion failed)
 - **9x** Timeout
 - **1x** Unhandled: Not callable: \\\
 - **1x** ReferenceError (undefined symbol)
@@ -19,13 +19,13 @@ Wall time: 37.7s
 
 | Category | Pass | Fail | Skip | Timeout | Total | Pass % |
 |---|---:|---:|---:|---:|---:|---:|
-| built-ins/String | 58 | 33 | 1123 | 9 | 1223 | 58.0% |
+| built-ins/String | 62 | 29 | 1123 | 9 | 1223 | 62.0% |
 
 ## Per-category top failures (min 10 runnable, worst first)
 
-### built-ins/String  (58/100 — 58.0%)
+### built-ins/String  (62/100 — 62.0%)
 
-- **28x** Test262Error (assertion failed)
+- **24x** Test262Error (assertion failed)
 - **9x** Timeout
 - **1x** Unhandled: Not callable: \\\
 - **1x** ReferenceError (undefined symbol)