js-on-sx: String(arr) honours Array.prototype.toString overrides

Was always emitting comma-joined via js-list-join, so user
mutations of Array.prototype.toString had no effect on String(arr)
/ "" + arr. Now look up the override via js-dict-get-walk and call
it on the list as this; fall back to (js-list-join v ",") when the
override doesn't return a string.
String fail count: 11 → 9. conformance.sh: 148/148.

lib/js/test262-scoreboard.md

@@ -1,26 +1,28 @@
 # test262 scoreboard
 
 Pinned commit: `d5e73fc8d2c663554fb72e2380a8c2bc1a318a33`
-Wall time: 179.5s
+Wall time: 141.9s
 
-**Total:** 86/99 runnable passed (86.9%). Raw: pass=86 fail=11 skip=1 timeout=2 total=100.
+**Total:** 23/45 runnable passed (51.1%). Raw: pass=23 fail=21 skip=5 timeout=1 total=50.
 
 ## Top failure modes
 
-- **10x** Test262Error (assertion failed)
-- **2x** Timeout
-- **1x** SyntaxError (parse/unsupported syntax)
+- **18x** Test262Error (assertion failed)
+- **2x** TypeError: not a function
+- **1x** Timeout
+- **1x** Unhandled: Not callable: {:2 43 :1 42 :0 41 :length 3}\
 
 ## Categories (worst pass-rate first, min 10 runnable)
 
 | Category | Pass | Fail | Skip | Timeout | Total | Pass % |
 |---|---:|---:|---:|---:|---:|---:|
-| built-ins/String | 86 | 11 | 1 | 2 | 100 | 86.9% |
+| built-ins/Array | 23 | 21 | 5 | 1 | 50 | 51.1% |
 
 ## Per-category top failures (min 10 runnable, worst first)
 
-### built-ins/String  (86/99 — 86.9%)
+### built-ins/Array  (23/45 — 51.1%)
 
-- **10x** Test262Error (assertion failed)
-- **2x** Timeout
-- **1x** SyntaxError (parse/unsupported syntax)
+- **18x** Test262Error (assertion failed)
+- **2x** TypeError: not a function
+- **1x** Timeout
+- **1x** Unhandled: Not callable: {:2 43 :1 42 :0 41 :length 3}\