Replace two-node-udp with Docker network test harness, fix dead code warnings

Replace examples/two-node-udp/ (netns-based) with examples/docker-network/
(Docker compose, 5 nodes, mesh + chain topologies). The new harness uses
auto-detecting build script and includes SVG topology diagrams.

Remove unused get_session_mut, remote_addr(), remote_pubkey() methods.
Gate test-only accessors with #[cfg(test)]. Zero warnings in release build.

src/node/mod.rs

@@ -806,16 +806,13 @@ impl Node {
     // === End-to-End Sessions ===
 
     /// Get a session by remote NodeAddr.
+    #[cfg(test)]
     pub(crate) fn get_session(&self, remote: &NodeAddr) -> Option<&SessionEntry> {
         self.sessions.get(remote)
     }
 
-    /// Get a mutable session by remote NodeAddr.
-    pub(crate) fn get_session_mut(&mut self, remote: &NodeAddr) -> Option<&mut SessionEntry> {
-        self.sessions.get_mut(remote)
-    }
-
     /// Remove a session.
+    #[cfg(test)]
     pub(crate) fn remove_session(&mut self, remote: &NodeAddr) -> Option<SessionEntry> {
         self.sessions.remove(remote)
     }

src/node/session.rs

@@ -42,12 +42,15 @@ impl EndToEndState {
 /// The state is `None` only transiently during handler processing.
 pub(crate) struct SessionEntry {
     /// Remote node's address (session table key).
+    #[allow(dead_code)]
     remote_addr: NodeAddr,
     /// Remote node's static public key (for Noise IK).
+    #[allow(dead_code)]
     remote_pubkey: PublicKey,
     /// Current session state. `None` only during state transitions.
     state: Option<EndToEndState>,
     /// When the session was created (Unix milliseconds).
+    #[cfg_attr(not(test), allow(dead_code))]
     created_at: u64,
     /// Last activity timestamp (Unix milliseconds).
     last_activity: u64,
@@ -70,16 +73,6 @@ impl SessionEntry {
         }
     }
 
-    /// Get the remote node address.
-    pub(crate) fn remote_addr(&self) -> &NodeAddr {
-        &self.remote_addr
-    }
-
-    /// Get the remote node's public key.
-    pub(crate) fn remote_pubkey(&self) -> &PublicKey {
-        &self.remote_pubkey
-    }
-
     /// Get the current session state.
     pub(crate) fn state(&self) -> &EndToEndState {
         self.state.as_ref().expect("session state taken but not restored")
@@ -114,11 +107,13 @@ impl SessionEntry {
     }
 
     /// Get creation time.
+    #[cfg(test)]
     pub(crate) fn created_at(&self) -> u64 {
         self.created_at
     }
 
     /// Get last activity time.
+    #[cfg(test)]
     pub(crate) fn last_activity(&self) -> u64 {
         self.last_activity
     }