When

An event matcher used to scope reducers, middleware, and effects. Exactly one variant is expected, mirroring the TypeScript union: - `{"any": True}` — match every event. - `{"keys": [(channel, type), ...]}` — match specific event keys. - `{"channel": name}` — match every event on one channel. - `{"channels": [name, ...]}` — match every event on any listed channel.