New features:
- .map() / .filter() for arrays with lambda support
- @validate decorator for argument validation
- fs.open() / f.read() / f.write() / f.close() file handles
- with X in Y { } context manager for auto-closing resources

All features work in both Bash and @awk codegen.
Added 18 new pytest tests for the new functionality.
Updated documentation (LANGUAGE_SPEC.md, README.md, README_ru.md).