js-number-to-string did (str n), which gives OCaml-native "inf"/"-inf"/
"nan"/"-nan" strings. JS spec requires "Infinity"/"-Infinity"/"NaN".
Fix: cond-check js-number-is-nan, and =infinity-value first, fall
through to (str n) for finite.
Unit 521/522, slice 148/148 unchanged.
String scoreboard: 34/100 → 38/100 (+4, S15.5.1.1_A1_T11/T12 family —
String(1/0)/String(-1/0)/String(0/0)).