feat: scaffold CQRS sample with DDD and package-by-feature design (Helidon MP + MongoDB)

[Copilot]

Repository contained only a README. This PR builds the full project from scratch, applying Domain-Driven Design and package-by-feature organization throughout.

Structure

Each bounded context owns its full vertical slice — no cross-feature leakage:

com.soujava.helidon.cqrs
├── shared/              ← CQRS bus abstractions (Command, Query, buses, AggregateRoot, DomainEvent)
└── order/               ← Order bounded context
    ├── api/             ← JAX-RS resource, request DTOs, DomainExceptionMapper
    ├── command/         ← CreateOrder / ConfirmOrder / CancelOrder + handlers
    ├── query/           ← FindOrderById / FindAllOrders + handlers, OrderView read model, OrderViewMapper
    ├── domain/          ← Order aggregate, OrderId / OrderStatus value objects, OrderItem, OrderRepository interface
    │   └── event/       ← OrderCreatedEvent, OrderConfirmedEvent, OrderCancelledEvent
    ├── infrastructure/  ← MongoClientProducer, MongoOrderRepository
    └── OrderHandlerRegistrar.java

DDD application

• Order is the aggregate root — all invariants enforced internally, state changes only through named business methods (confirm(), cancel())
• OrderId / OrderStatus are immutable value objects
• Domain events (OrderCreatedEvent, etc.) are registered on the aggregate and collected via AggregateRoot
• OrderRepository interface lives in the domain; MongoDB implementation lives in infrastructure — domain has zero persistence imports

CQRS wiring

Handlers register themselves at CDI startup via OrderHandlerRegistrar (@Observes @Initialized), keeping the CommandBus / QueryBus decoupled from any feature. OrderResource dispatches only through the bus interfaces — no domain logic in the API layer.

Key decisions

• OrderViewMapper centralises aggregate → read model conversion, shared by both query handlers
• DomainExceptionMapper (@Provider) maps IllegalArgumentException → 400, IllegalStateException → 409, logs unexpected errors server-side
• OrderResource.createOrder() validates payload nulls before dispatch to give clean API errors
• Config (mongodb.uri, mongodb.database, server.port) externalised via MicroProfile Config