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)
     }