⭐ EventBus

Done. All 8 source files (Abstractions/, Events/, Extensions/, GlobalUsings.cs) added and reviewed for correctness and SOLID/DRY/composition.

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 instead of calling each other directly.

Most files needed no changes — the abstractions stay minimal and ISP-compliant. A later re-review of IntegrationEvent.cs (matching the depth applied to EventBusRabbitMQ) found two real, if minor, issues: a [JsonInclude] attribute that was dead weight under both serialization paths, and two properties left mutable (set) when nothing in the repo ever mutates them post-construction — locked to init. Full detail and commit hashes in todo.md's EventBus section.

Test coverage complete too: tests/EventBus.UnitTests, 16 passing tests across all 6 applicable files (the 2 pure interfaces need none) — see Testing.