feat: realtime channel subscriptions in @primora/client

createRealtimeClient opens the project SSE stream via fetch +
ReadableStream (EventSource cannot send X-API-Key), with capped
exponential-backoff reconnect and state callbacks. channel() filters by
event namespace — channel("documents").on("created", cb). Presence and
client-broadcast deliberately deferred.

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

packages/client/src/index.ts

@@ -1,4 +1,14 @@
 export type PrimoraEventType = "error" | "metric" | "log" | "heartbeat" | "event";
+
+export {
+  createRealtimeClient,
+  type RealtimeChannel,
+  type RealtimeClient,
+  type RealtimeClientOptions,
+  type RealtimeEvent,
+  type RealtimeHandler,
+  type RealtimeState,
+} from "./realtime.js";
 export type PrimoraComponentKind =
   | "frontend" | "backend" | "database" | "android" | "desktop" | "web" | "other";