In this commit we add FindPath, a BFS-based shsortest-path
algorithm that finds routes through the channel graph for
onion messages. The search filters nodes by the
OnionMessage feature bits (38/39).
We also add a unit tests covering: direct neighbor routing,
multi-hop paths, feature-bit filtering, missing destination
nodes, destination without onion support, max hop limits,
cycle handling, and shortest-path selection.
choice of BFS is because there isn't any weight involve.
Block forwarding of an onion message when the resolved next hop is the
same peer that delivered it. Such a forward would immediately bounce the
message back over the very connection it arrived on, which is never
useful and can be abused to amplify traffic against a peer.
The check runs after the routing action is resolved, so both direct
next-node-ID and SCID-resolved paths are covered. A new
`ErrSamePeerCycle` is returned (and logged at warn level) when a cycle
is detected.
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
