ChainEvent

One thing that happened to the chain, as a backend reports it.

Deliberately normalised rather than backend-shaped. If a follower emitted its own wire types the driver below would need a branch per backend and the sharing would evaporate — the whole point is that everything downstream of this type is written once. Backend-shaped payloads are still worth keeping where they earn their place: recorded at the HTTP/WebSocket boundary, as test fixtures.

Public, since EmulatorBase.diff returns it, spec [SY-1].