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/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;