js-list-set! was a no-op for the length key. Added a clause that
pads with js-undefined via js-pad-list! when target > current.
Truncation skipped: the pop-last! SX primitive doesn't actually
mutate the list (length unchanged after the call), so no clean
way to shrink in place from SX. Extension covers common cases.
built-ins/Array: 16/45 → 17/45. conformance.sh: 148/148.