Implement a robust journal system featuring a type-safe message
registry and an MQTT-backed stream store.
- Add `registry.ts` to manage message type definitions, Zod schemas,
and custom reducers.
- Implement `journalStream.ts` to manage MQTT connections, session
lifecycle, and reactive message state.
- Add built-in message types for `narrative`, `intent`, `roll`,
and `article`.
- Provide utilities for message validation, optimistic local updates,
and message reversion.
