Split protocol.rs into protocol/ directory, standardize imports, clean lib.rs

- Replace use super::*; in node/lifecycle.rs with explicit imports,
  remove 10 resulting unused imports from node/mod.rs
- Split protocol.rs (1838 lines) into protocol/ directory:
  mod.rs (re-exports), error.rs, link.rs, tree.rs, filter.rs,
  discovery.rs, session.rs — each with co-located tests
- Remove unused flat re-exports from lib.rs for internal utility
  modules (wire, index, rate_limit, icmp, noise, tun)
- 316 tests pass, zero warnings

src/node/lifecycle.rs

@@ -1,7 +1,15 @@
 //! Node lifecycle management: start, stop, and peer connection initiation.
 
-use super::*;
+use super::{Node, NodeError, NodeState};
+use crate::peer::PeerConnection;
 use crate::protocol::{Disconnect, DisconnectReason};
+use crate::transport::{packet_channel, Link, LinkDirection, TransportAddr};
+use crate::tun::{run_tun_reader, shutdown_tun_interface, TunDevice, TunState};
+use crate::wire::build_msg1;
+use crate::{NodeAddr, PeerIdentity};
+use std::thread;
+use std::time::Duration;
+use tracing::{debug, info, warn};
 
 impl Node {
     /// Initiate connections to configured static peers.