Function build_shuffle_sender 

async fn build_shuffle_sender(
    node_id: u64,
    discovery: &DiscoveryImpl,
    advertise_addr: String,
) -> ShuffleSender

Build an outbound shuffle sender. When gossip discovery is active, publish advertise_addr under SHUFFLE_ADDR_KEY so peers find us, and give the sender a KV handle for reverse lookup. Static discovery has no KV tier, so we hand back a bare sender — peers must be registered explicitly by whatever sets up the shuffle topology.