Opt-in for-in loops

Date: 2026-06-02 Area: engine

Added opt-in for...in JavaScript compatibility behind --compat-for-in-loop / "compat-for-in-loop" / cfForIn. The flag remains separate from --compat-traditional-for-loop because for...in uses property-name enumeration rather than counted-loop control flow. It stays off by default with the existing parser warning/no-op behavior and supports enumerable string own and inherited keys in interpreter and bytecode modes. language.md.