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/scripts/build.sh

@@ -0,0 +1,24 @@
+#!/bin/bash
+# Build the FIPS binary and copy it to the docker build context.
+set -e
+
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
+DOCKER_DIR="$(cd "$SCRIPT_DIR/.." && pwd)"
+
+# Find project root (directory containing Cargo.toml)
+PROJECT_ROOT="$(cd "$DOCKER_DIR/../.." && pwd)"
+if [ ! -f "$PROJECT_ROOT/Cargo.toml" ]; then
+    echo "Error: Cannot find Cargo.toml at $PROJECT_ROOT" >&2
+    echo "Expected layout: <project-root>/examples/docker-network/scripts/build.sh" >&2
+    exit 1
+fi
+
+echo "Building FIPS (release)..."
+cargo build --release --manifest-path="$PROJECT_ROOT/Cargo.toml"
+
+echo "Copying binary to docker context..."
+cp "$PROJECT_ROOT/target/release/fips" "$DOCKER_DIR/fips"
+
+echo "Done. Binary at $DOCKER_DIR/fips"
+echo ""
+echo "Next: cd $DOCKER_DIR && docker compose --profile mesh build"