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.

examples/docker-network/Dockerfile

@@ -0,0 +1,11 @@
+FROM debian:bookworm-slim
+
+RUN apt-get update && \
+    apt-get install -y --no-install-recommends \
+        iproute2 iputils-ping dnsutils && \
+    rm -rf /var/lib/apt/lists/*
+
+COPY fips /usr/local/bin/
+RUN chmod +x /usr/local/bin/fips
+
+ENTRYPOINT ["fips", "--config", "/etc/fips/fips.yaml"]