js-on-sx: bail out of array set/length at 2^32-1 instead of padding

arr[4294967295] = 'x' and arr.length = 4294967295 were padding
the SX list with js-undefined for ~4 billion entries — instant
timeout. Per ES spec, indices >= 2^32-1 aren't array indices
anyway (regular properties, which we can't store on lists).
Added (>= i 4294967295) bail clauses to js-list-set! and the
length setter.
built-ins/Array: 21/45 → 23/45 (5 timeouts → 2).
conformance.sh: 148/148.

lib/js/test262-scoreboard.md

@@ -1,28 +1,28 @@
 # test262 scoreboard
 
 Pinned commit: `d5e73fc8d2c663554fb72e2380a8c2bc1a318a33`
-Wall time: 123.5s
+Wall time: 51.3s
 
-**Total:** 21/45 runnable passed (46.7%). Raw: pass=21 fail=17 skip=5 timeout=7 total=50.
+**Total:** 23/45 runnable passed (51.1%). Raw: pass=23 fail=20 skip=5 timeout=2 total=50.
 
 ## Top failure modes
 
-- **14x** Test262Error (assertion failed)
-- **7x** Timeout
+- **17x** Test262Error (assertion failed)
 - **2x** TypeError: not a function
+- **2x** 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/Array | 21 | 17 | 5 | 7 | 50 | 46.7% |
+| built-ins/Array | 23 | 20 | 5 | 2 | 50 | 51.1% |
 
 ## Per-category top failures (min 10 runnable, worst first)
 
-### built-ins/Array  (21/45 — 46.7%)
+### built-ins/Array  (23/45 — 51.1%)
 
-- **14x** Test262Error (assertion failed)
-- **7x** Timeout
+- **17x** Test262Error (assertion failed)
 - **2x** TypeError: not a function
+- **2x** Timeout
 - **1x** Unhandled: Not callable: {:2 43 :1 42 :0 41 :length 3}\