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.test.ts

@@ -0,0 +1,34 @@
+import { describe, it, expect } from "vitest";
+import { z } from "zod";
+import { InMemoryRealtimeBroadcaster } from "@/in-memory-realtime-broadcaster";
+import { defineRealtimeChannel } from "@/realtime-channel";
+
+const ch = defineRealtimeChannel("a.b", z.object({ x: z.number() }).strict(), {
+  scope: "public",
+});
+
+describe("InMemoryRealtimeBroadcaster", () => {
+  it("validates payload via the descriptor schema", async () => {
+    const b = new InMemoryRealtimeBroadcaster();
+    await expect(
+      b.broadcast(ch, { x: "not a number" } as never),
+    ).rejects.toThrow();
+  });
+
+  it("delivers to subscribers in order", async () => {
+    const b = new InMemoryRealtimeBroadcaster();
+    const got: number[] = [];
+    b.subscribe(ch, async (p) => {
+      got.push(p.x);
+    });
+    await b.broadcast(ch, { x: 1 });
+    await b.broadcast(ch, { x: 2 });
+    expect(got).toEqual([1, 2]);
+  });
+
+  it("does nothing when no subscribers", async () => {
+    const b = new InMemoryRealtimeBroadcaster();
+    await b.broadcast(ch, { x: 1 });
+    // does not throw
+  });
+});