Akka Hosting Actor Patterns

This is a solid architectural pattern for building Akka.NET entity actors that work identically in unit tests and production. The key insight is using GenericChildPerEntityParent to mimic cluster sharding semantics locally, so you write your actor once and swap execution modes via configuration. The message extractor pattern keeps routing logic consistent whether you're running locally or in a real cluster. The Akka.Hosting extension methods are clean and the ITimeProvider abstraction solves the DateTime.Now testing problem elegantly. If you're building domain entities with Akka and tired of maintaining separate test and production actor setups, this pattern saves real work.