Implement IrohTransport and IrohAcceptor (feature-gated iroh)

Add iroh QUIC P2P transport using tokio::io::join for stream duplexing
per ADR-003. Default relay is n0's https://relay.iroh.network/ (ADR-009).
Proxy URL passed to Endpoint::builder (ADR-010). Integration test marked
#[ignore] for CI since it requires iroh relay connectivity.

crates/wraith-core/src/transport/mod.rs

@@ -1,6 +1,10 @@
 mod tcp;
+#[cfg(feature = "iroh")]
+mod iroh_transport;
 
 pub use tcp::{TcpAcceptor, TcpTransport};
+#[cfg(feature = "iroh")]
+pub use iroh_transport::{IrohAcceptor, IrohTransport};
 
 use std::net::SocketAddr;