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/error.rs

@@ -0,0 +1,31 @@
+//! Protocol error types.
+
+use thiserror::Error;
+
+/// Errors related to protocol message handling.
+#[derive(Debug, Error)]
+pub enum ProtocolError {
+    #[error("invalid message type: 0x{0:02x}")]
+    InvalidMessageType(u8),
+
+    #[error("message too short: expected at least {expected}, got {got}")]
+    MessageTooShort { expected: usize, got: usize },
+
+    #[error("message too long: max {max}, got {got}")]
+    MessageTooLong { max: usize, got: usize },
+
+    #[error("invalid signature")]
+    InvalidSignature,
+
+    #[error("unsupported protocol version: {0}")]
+    UnsupportedVersion(u8),
+
+    #[error("malformed message: {0}")]
+    Malformed(String),
+
+    #[error("hop limit exceeded")]
+    HopLimitExceeded,
+
+    #[error("ttl expired")]
+    TtlExpired,
+}