Function loopback_path_stream 

fn loopback_path_stream<U>(
    cfg: ProberConfig,
    graph: U,
) -> impl Stream<Item = (Vec<OffchainPublicKey>, PathId)>
where
    U: NetworkGraphTraverse<NodeId = OffchainPublicKey> + Clone + Send + Sync + 'static,

Stream that cycles through 1-, 2-, and 3-hop loopback paths with weighted shuffle.

Shared by both cover traffic and intermediate probing — the caller wraps each emitted item into the appropriate outer type.