Replace @repeaterjs/repeater dependency with inlined Repeater class

Port the core Repeater class (push/stop executor, async iterator protocol,
overflow protection) from @repeaterjs/repeater into src/repeater.ts, stripping
buffers and combinators we don't use. This removes the 7-year-old dependency
while keeping the same API surface for all current and future adapters
(Redis, WebSocket, SSE, Iroh).

Also add the async utility research doc for reference.

src/create_pubsub.ts

@@ -24,7 +24,7 @@
  * SOFTWARE.
  */
 
-import { Repeater } from "@repeaterjs/repeater";
+import { Repeater } from "./repeater.js";
 import type { TypedEventTarget, TypedEvent } from "./types.js";
 
 export type PubSubPublishArgsByKey = {