js-pow-int 10 20 overflows int64 (10^20 > 2^63), so numeric literals
like 1e20 and 100000000000000000000 were parsing as
-1457092405402533888. The pow primitive uses float-domain
exponentiation and produces 1e+20 correctly. Single call swap in
js-num-from-string. built-ins/String (with --restart-every 1):
67/99 → 70/99. conformance.sh: 148/148.