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.

package-lock.json

@@ -8,9 +8,6 @@
       "name": "@alkdev/pubsub",
       "version": "0.1.0",
       "license": "MIT OR Apache-2.0",
-      "dependencies": {
-        "@repeaterjs/repeater": "^3.0.0"
-      },
       "devDependencies": {
         "@types/node": "^22.0.0",
         "@vitest/coverage-v8": "^3.2.4",
@@ -632,12 +629,6 @@
         "node": ">=14"
       }
     },
-    "node_modules/@repeaterjs/repeater": {
-      "version": "3.0.6",
-      "resolved": "https://registry.npmjs.org/@repeaterjs/repeater/-/repeater-3.0.6.tgz",
-      "integrity": "sha512-Javneu5lsuhwNCryN+pXH93VPQ8g0dBX7wItHFgYiwQmzE1sVdg5tWHiOgHywzL2W21XQopa7IwIEnNbmeUJYA==",
-      "license": "MIT"
-    },
     "node_modules/@rollup/rollup-android-arm-eabi": {
       "version": "4.60.2",
       "resolved": "https://registry.npmjs.org/@rollup/rollup-android-arm-eabi/-/rollup-android-arm-eabi-4.60.2.tgz",