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/index.ts

@@ -1,4 +1,4 @@
 export { createPubSub, type PubSub, type PubSubConfig, type PubSubEvent, type PubSubEventTarget, type PubSubPublishArgsByKey } from "./create_pubsub.js";
 export { type TypedEvent, type TypedEventTarget, type TypedEventListener, type TypedEventListenerObject, type TypedEventListenerOrEventListenerObject } from "./types.js";
 export { filter, map, pipe } from "./operators.js";
-export { Repeater } from "@repeaterjs/repeater";
+export { Repeater, RepeaterOverflowError, type Push, type Stop, type RepeaterExecutor, type RepeaterBuffer } from "./repeater.js";