Refactor: Modularize evaluator, unified parser, and 100% test compatibility
- Split function_parser.go into expressions.go, function_eval.go, and function_parser.go
- Implement unified ExpressionEvaluator for all predicate types
- Add Debug flag to Options for detailed execution tracing
- Fix NodeSet truthiness and text node dot predicate parsing
- Fix nested predicate parsing bug in FunctionParser
- Achieve 100% pass rate on comprehensive test suite