js-on-sx: js-to-string emits comma-joined elements for SX lists

String([1,2,3]) was returning "(1 2 3)" (the SX (str v) fallback in
js-to-string fell through for SX lists). Replaced the fallback with
a list-typed branch that delegates to (js-list-join v ","). Fixes
String(arr), "" + arr, and any implicit array-to-string coercion.
built-ins/String: 65/99 → 67/99. conformance.sh: 148/148.

lib/js/test262-scoreboard.md

@@ -1,15 +1,14 @@
 # test262 scoreboard
 
 Pinned commit: `d5e73fc8d2c663554fb72e2380a8c2bc1a318a33`
-Wall time: 353.1s
+Wall time: 580.2s
 
-**Total:** 65/99 runnable passed (65.7%). Raw: pass=65 fail=28 skip=1 timeout=6 total=100.
+**Total:** 67/99 runnable passed (67.7%). Raw: pass=67 fail=19 skip=1 timeout=13 total=100.
 
 ## Top failure modes
 
-- **16x** Test262Error (assertion failed)
-- **8x** TypeError: not a function
-- **6x** Timeout
+- **15x** Test262Error (assertion failed)
+- **13x** Timeout
 - **2x** Unhandled: Not callable: \\\
 - **1x** ReferenceError (undefined symbol)
 - **1x** SyntaxError (parse/unsupported syntax)
@@ -18,14 +17,14 @@ Wall time: 353.1s
 
 | Category | Pass | Fail | Skip | Timeout | Total | Pass % |
 |---|---:|---:|---:|---:|---:|---:|
-| built-ins/String | 65 | 28 | 1 | 6 | 100 | 65.7% |
+| built-ins/String | 67 | 19 | 1 | 13 | 100 | 67.7% |
 
 ## Per-category top failures (min 10 runnable, worst first)
 
-### built-ins/String  (65/99 — 65.7%)
+### built-ins/String  (67/99 — 67.7%)
 
-- **16x** Test262Error (assertion failed)
-- **8x** TypeError: not a function
-- **6x** Timeout
+- **15x** Test262Error (assertion failed)
+- **13x** Timeout
 - **2x** Unhandled: Not callable: \\\
 - **1x** ReferenceError (undefined symbol)
+- **1x** SyntaxError (parse/unsupported syntax)