Introduce hands-on exercises with tests covering:
* part1: MoneyExercises, ArithmeticExercises, RoundingExercises,
FormattingExercises, ParsingExercises, IntegrationExercises
* part2: CurrencyConversionExercises, MonetaryOperatorExercises,
MonetaryQueryExercises, MonetaryContextExercises,
RoundingStrategyExercises
* part3: OrderExercises (end-to-end order workflow)
Use incomplete production methods with TODOs and complete JUnit 5 tests
(AssertJ only). Tests focus on observable behavior and avoid fragile
assertions (e.g., no fixed FX rates). Ensure clear progression from
basics to advanced concepts and final integration task.
