Discovery protocol: LookupRequest/LookupResponse handlers

Implement the coordinate discovery protocol with flood-based lookup
and reverse-path response routing.

Wire format: LookupRequest (0x30) encode/decode with TTL, visited
bloom filter, origin coords. LookupResponse (0x31) encode/decode
with target coords and Schnorr proof signature.

Handler logic: request dedup by request_id, visited filter loop
prevention, TTL enforcement, lazy purge of expired entries (10s).
Response routing: originator caches route in route_cache, transit
nodes reverse-path forward via recent_requests.

Node state: RecentRequest struct, route_cache (RouteCache), and
recent_requests map for dedup + reverse-path forwarding.

13 handler tests (9 unit + 4 integration) plus 4 protocol tests.
392 tests pass, clean build.

src/protocol/session.rs

@@ -76,7 +76,7 @@ impl fmt::Display for SessionMessageType {
 ///
 /// Session-layer messages serialize coordinates as NodeAddr arrays (16 bytes each),
 /// without the sequence/timestamp metadata used by the tree gossip protocol.
-fn encode_coords(coords: &TreeCoordinate, buf: &mut Vec<u8>) {
+pub(crate) fn encode_coords(coords: &TreeCoordinate, buf: &mut Vec<u8>) {
     let addrs: Vec<&NodeAddr> = coords.node_addrs().collect();
     let count = addrs.len() as u16;
     buf.extend_from_slice(&count.to_le_bytes());
@@ -88,7 +88,7 @@ fn encode_coords(coords: &TreeCoordinate, buf: &mut Vec<u8>) {
 /// Decode a TreeCoordinate from address-only wire format.
 ///
 /// Returns the decoded coordinate and the number of bytes consumed.
-fn decode_coords(data: &[u8]) -> Result<(TreeCoordinate, usize), ProtocolError> {
+pub(crate) fn decode_coords(data: &[u8]) -> Result<(TreeCoordinate, usize), ProtocolError> {
     if data.len() < 2 {
         return Err(ProtocolError::MessageTooShort {
             expected: 2,