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.

src/node/mod.rs

@@ -18,20 +18,17 @@ use crate::index::IndexAllocator;
 use crate::peer::{ActivePeer, PeerConnection};
 use crate::rate_limit::HandshakeRateLimiter;
 use crate::transport::{
-    packet_channel, Link, LinkDirection, LinkId, PacketRx, PacketTx,
-    TransportAddr, TransportHandle, TransportId,
+    Link, LinkId, PacketRx, PacketTx, TransportAddr, TransportHandle, TransportId,
 };
 use crate::transport::udp::UdpTransport;
 use crate::tree::TreeState;
-use crate::tun::{run_tun_reader, shutdown_tun_interface, TunDevice, TunError, TunState, TunTx};
-use crate::wire::{build_encrypted, build_msg1};
-use crate::{Config, ConfigError, Identity, IdentityError, NodeAddr, PeerIdentity};
+use crate::tun::{TunError, TunState, TunTx};
+use crate::wire::build_encrypted;
+use crate::{Config, ConfigError, Identity, IdentityError, NodeAddr};
 use std::collections::HashMap;
 use std::fmt;
-use std::thread::{self, JoinHandle};
-use std::time::Duration;
+use std::thread::JoinHandle;
 use thiserror::Error;
-use tracing::{debug, info, warn};
 
 /// Errors related to node operations.
 #[derive(Debug, Error)]

src/node/tests/mod.rs

@@ -1,6 +1,7 @@
 use super::*;
 use crate::index::SessionIndex;
-use crate::transport::{LinkDirection, TransportAddr};
+use crate::transport::{packet_channel, LinkDirection, TransportAddr};
+use crate::PeerIdentity;
 use std::time::Duration;
 
 mod bloom;