⭐ EventBus

In progress, honestly: EventBus.csproj is added (a2e37eb), but its 8 source files (Abstractions/, Events/, Extensions/, GlobalUsings.cs) aren't yet — this page will get real content once they land.

EventBus is the abstractions layer for this repo's event-driven integration model — see Architecture Overview for why services publish/consume integration events through this and EventBusRabbitMQ (RabbitMQ-backed, not started) instead of calling each other directly. Next in the migration order after this: EventBusRabbitMQ, then eShop.ServiceDefaults, then IntegrationEventLogEF — the rest of the foundation layer everything else depends on.