feat(soak): RoomCoordinator with host→joiners handoff

Lazy Deferred per roomId with a timeout on await. Lets concurrent
joiner sessions block until their host announces the room code
without polling or page scraping.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/soak/core/room-coordinator.ts

@@ -0,0 +1,42 @@
+/**
+ * RoomCoordinator — tiny host→joiners handoff primitive.
+ *
+ * Lazy Deferred per roomId. `announce` resolves the promise; `await`
+ * blocks until the promise resolves or a per-call timeout fires.
+ * Rooms are keyed by string; each key has at most one Deferred.
+ */
+
+import { deferred, Deferred } from './deferred';
+import type { RoomCoordinatorApi } from './types';
+
+export class RoomCoordinator implements RoomCoordinatorApi {
+  private rooms = new Map<string, Deferred<string>>();
+
+  announce(roomId: string, code: string): void {
+    this.getOrCreate(roomId).resolve(code);
+  }
+
+  async await(roomId: string, timeoutMs: number = 30_000): Promise<string> {
+    const d = this.getOrCreate(roomId);
+    let timer: NodeJS.Timeout | undefined;
+    const timeout = new Promise<never>((_, reject) => {
+      timer = setTimeout(() => {
+        reject(new Error(`RoomCoordinator: room "${roomId}" timed out after ${timeoutMs}ms`));
+      }, timeoutMs);
+    });
+    try {
+      return await Promise.race([d.promise, timeout]);
+    } finally {
+      if (timer) clearTimeout(timer);
+    }
+  }
+
+  private getOrCreate(roomId: string): Deferred<string> {
+    let d = this.rooms.get(roomId);
+    if (!d) {
+      d = deferred<string>();
+      this.rooms.set(roomId, d);
+    }
+    return d;
+  }
+}