Add Casino Engine to Hexagonal Architecture Samples

[nekzabirov]

Summary

Adds nekzabirov/IGaming-Game-Engine to docs/hexagonal-architecture.md → Samples.

Why this fits the Hexagonal section

Casino Engine is a production-grade open-source iGaming/casino engine in Kotlin/Ktor structured strictly around the ports & adapters pattern:

• Domain layer has zero framework dependencies — aggregates, value objects, repository interfaces, sealed exception hierarchy
• Application layer declares ports (`IWalletPort`, `IPlayerLimitPort`, `FileAdapter`, `IEventPort`) and orchestrates use cases
• Infrastructure layer holds adapters: Exposed (PostgreSQL), RabbitMQ, S3, Redis, gRPC clients
• Three vendor aggregator adapters (Pragmatic Play, OneGameHub, Pateplay) implement the same `AggregatorAdapterProvider` port and are discovered at boot via Koin's `getAll()` — adding a fourth vendor is one new file + one Koin line

Production-tested on moonbet.casino and 2k.ua. Apache 2.0.

The samples section currently has examples in .NET, Node.js, and Java, but no Kotlin/Ktor reference at this scale.

Checklist

• One PR per category (this is the hexagonal-architecture.md PR; separate PRs for CQRS / EDA / gRPC categories are open / coming)
• Entry format matches existing: `user/repo - Description`
• Meaningful description
• Active and maintained