fix(builtins): prevent awk parser panic on multi-byte UTF-8 (#476)

## Summary
- Add `current_char()` and `advance()` helpers for char-boundary safe
parsing
- Replace all 66+ `chars().nth(self.pos)` calls with `current_char()`
- Fix string, regex, identifier, and comment parsing for multi-byte
chars
- Fix `matches_keyword` lookahead to use slice-based char access

Closes #395

Co-authored-by: Claude <noreply@anthropic.com>

Author: chaliy