feat(journal): implement message registry and stream store

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.

src/components/journal/index.ts

@@ -0,0 +1,35 @@
+/**
+ * Journal module — barrel export
+ */
+
+// Trigger message type registration
+import "./types";
+
+export {
+  registerMessageType,
+  getMessageType,
+  registeredTypes,
+  validatePayload,
+  canEmit,
+} from "./registry";
+export type { MessageTypeDef, StreamMessage } from "./registry";
+export {
+  connectStream,
+  disconnectStream,
+  hydrateFromServer,
+  sendMessage,
+  revertLatest,
+  canRevert,
+  visibleMessages,
+  useJournalStream,
+  journalStreamState,
+  createSession,
+  deleteSession,
+  sessions,
+  setMyName,
+} from "../stores/journalStream";
+export type {
+  JournalStreamState,
+  SessionMeta,
+  SessionManifest,
+} from "../stores/journalStream";