feat(rules): replace go templates with custom variable expansion

- Replace template syntax ({{ .Request.Method }}) with $-prefixed variables ($req_method)
- Implement custom variable parser with static ($req_method, $status_code) and dynamic ($header(), $arg(), $form()) variables
- Replace templateOrStr interface with templateString struct and ExpandVars methods
- Add parser improvements for reliable quote handling
- Add new error types: ErrUnterminatedParenthesis, ErrUnexpectedVar, ErrExpectOneOrTwoArgs
- Update all tests and help text to use new variable syntax
- Add comprehensive unit and benchmark tests for variable expansion

internal/route/rules/on.go

@@ -591,7 +591,7 @@ func parseOn(line string) (Checker, bool, gperr.Error) {
 
 	validArgs, err := checker.validate(args)
 	if err != nil {
-		return nil, false, err.Subject(subject).With(checker.help.Error())
+		return nil, false, err.With(checker.help.Error())
 	}
 
 	checkFunc := checker.builder(validArgs)