feat: Add FoldPostfix command and Haxe grammar for for-range loops

- Add FoldPostfix command to runtime for handling postfix expression chains
- Implement fold_postfix JSON command parsing in ZynPEG runtime
- Add create_range and apply_postfix methods to AstHostFunctions trait
- Add haxe.zyn grammar with support for classes, methods, control flow
- Desugar for-in loops with Range expressions to while loops
- Fix stack pollution issue where unrecognized commands left stale values

This enables parsing Haxe for loops like `for (j in 0...5) { }` and
correctly desugaring them to equivalent while loops.

Author: darmie