[Javadoc] Add @since attribute.
[Javadoc] Refine documentation.
[Validator] Rename method.
[Reflector] Make class final.
[Validator] Add Javadoc and reorder methods.
[Trivial] Replace 'and' with 'ampersand'.
[Refactor] Separate validation and callback logic.
[Validator] Enable asynchronous validation.
[ValidationError] Replace 'Rule' with 'List<Rule>'.