[Prototype] Rewrite JSONRPCConnection as an actor with AsyncStream

[rintaro]

Experimenting.

Three AsyncStreams.

AsyncStream<OutgoingItem> (outgoingContinuation)
Serializes all outgoing work items (notifications, requests, replies) from any caller into the actor. Its Continuation is a let property, making send/sendReply fully nonisolated — callers never have to hop to the actor just to enqueue a message. The fire-and-forget actor task drains it in strict FIFO order, guaranteeing message ordering without a dispatch queue.

AsyncStream<Data> (sendContinuation)
Decouples the actor-isolated encoding step from the blocking sendFD.write. The actor encodes a message and yields the bytes synchronously; the Task.detached send loop picks them up and blocks the thread waiting on I/O, without ever touching the actor. This is why two "send" streams are needed rather than one: the blocking write must not hold the actor's executor.

AsyncStream<Data> (receiveStream, created in _start)
Same idea on the read side. The Task.detached reader blocks on receiveFD.read, yields complete framed messages, and the actor-isolated receiveLoopTask picks them up to decode and dispatch. The stream is the hand-off point between the blocking I/O thread and the actor.