Generic cross-product event bus

A new platform-wide event bus (topic registry, product subscribers) for cross-product reactions.

Why not

The tRPC mutation path already is the event taxonomy — achievementPlugin proves it: any product mutation is addressable by path string with zero registration. A second bus would duplicate that with infrastructure (topics, delivery, replay) that has no consumer beyond achievements today. If a real second consumer appears, extend the existing middleware pattern rather than building a bus.