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/protocol/mod.rs

@@ -0,0 +1,48 @@
+//! FIPS Protocol Messages
+//!
+//! Wire format definitions for FIPS protocol communication across two layers:
+//!
+//! ## Link Layer (peer-to-peer, hop-by-hop)
+//!
+//! Messages exchanged between directly connected peers over Noise-encrypted
+//! links. Includes spanning tree gossip, bloom filter propagation, discovery
+//! protocol, and forwarding of session-layer datagrams.
+//!
+//! Link-layer peer authentication uses Noise IK (see `noise.rs`), which
+//! establishes the encrypted channel before any of these messages are sent.
+//!
+//! ## Session Layer (end-to-end, between FIPS addresses)
+//!
+//! Messages exchanged between source and destination FIPS nodes, encrypted
+//! with session keys that intermediate nodes cannot read. Includes session
+//! establishment, IPv6 datagram encapsulation, and routing errors.
+//!
+//! Session-layer datagrams are carried as opaque payloads through the link
+//! layer, encrypted end-to-end independently of per-hop link encryption.
+
+mod discovery;
+mod error;
+mod filter;
+mod link;
+mod session;
+mod tree;
+
+// Re-export all public types at protocol:: level
+pub use error::ProtocolError;
+pub use link::{
+    Disconnect, DisconnectReason, HandshakeMessageType, LinkMessageType, SessionDatagram,
+};
+pub use tree::TreeAnnounce;
+pub use filter::FilterAnnounce;
+pub use discovery::{LookupRequest, LookupResponse};
+pub use session::{
+    CoordsRequired, DataFlags, DataPacket, PathBroken, SessionAck, SessionFlags,
+    SessionMessageType, SessionSetup, DATA_FLAG_COORDS_PRESENT, DATA_HEADER_SIZE,
+};
+
+/// Protocol version for message compatibility.
+pub const PROTOCOL_VERSION: u8 = 1;
+
+// Legacy type alias re-export
+#[allow(deprecated)]
+pub use link::MessageType;