Offload functions to worker threads with shared memory primitives for Node.js.

feat(serve): listen() worker-side adapter with drain semantics master

Also adds PushChannel<T>/pushChannel() as a push-based async-iterable channel (with .send()/.close()), required by listen() and serverThreads. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>