Introduce a fat OnionPeerActor that handles the full onion message
processing pipeline for each peer connection. The actor decodes incoming
onion messages, determines the routing action (forward or deliver),
executes the action via PeerMessageSender, and dispatches updates to
subscribers via OnionMessageUpdateDispatcher.
Key components:
- OnionRouter interface abstracting sphinx router operations
- PeerMessageSender interface for forwarding to other peers
- OnionMessageUpdateDispatcher interface for subscriber notifications
- OnionActorFactory for spawning per-peer actors with shared deps
- Full test suite calling Receive() directly with NoOpReplayLog
