feat(core-realtime): scaffold realtime optional core

Generator-emitted scaffold (pnpm turbo gen core-package realtime) plus
the story-00-precedent coverage repairs (coverage provider devDep,
symbols.ts exclude + tested allowlist mirror) and three minimal tests
covering generator-emitted realtime code the template suite misses.
Squash of 31d85e0 + review-fix cf11b38.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016j8z4VHjedXDTjEDNg7qHK

packages/core-realtime/src/in-memory-realtime-broadcaster.ts

@@ -0,0 +1,28 @@
+import type { z } from "zod";
+import type { IRealtimeBroadcaster } from "./realtime-broadcaster.interface";
+import type { RealtimeChannelDescriptor } from "./realtime-channel";
+
+type Listener<T> = (payload: T) => Promise<void> | void;
+
+export class InMemoryRealtimeBroadcaster implements IRealtimeBroadcaster {
+  private readonly listeners = new Map<string, Listener<unknown>[]>();
+
+  async broadcast<T>(
+    descriptor: RealtimeChannelDescriptor<string, z.ZodType<T>>,
+    payload: T,
+  ): Promise<void> {
+    descriptor.schema.parse(payload);
+    const arr = this.listeners.get(descriptor.name) ?? [];
+    for (const l of arr) await l(payload);
+  }
+
+  // Test-friendly: lets unit tests subscribe directly without a Socket.IO server.
+  subscribe<T>(
+    descriptor: RealtimeChannelDescriptor<string, z.ZodType<T>>,
+    listener: Listener<T>,
+  ): void {
+    const arr = this.listeners.get(descriptor.name) ?? [];
+    arr.push(listener as Listener<unknown>);
+    this.listeners.set(descriptor.name, arr);
+  }
+}