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"

examples/docker-network/scripts/ping-test.sh

@@ -0,0 +1,102 @@
+#!/bin/bash
+# End-to-end ping test between FIPS nodes via DNS resolution.
+# Usage: ./ping-test.sh [mesh|chain]
+#
+# Requires containers to be running:
+#   docker compose --profile mesh up -d
+#   ./scripts/ping-test.sh mesh
+set -e
+
+PROFILE="${1:-mesh}"
+COUNT=3
+TIMEOUT=10
+PASSED=0
+FAILED=0
+
+# Node identities
+NPUB_A="npub1sjlh2c3x9w7kjsqg2ay080n2lff2uvt325vpan33ke34rn8l5jcqawh57m"
+NPUB_B="npub1tdwa4vjrjl33pcjdpf2t4p027nl86xrx24g4d3avg4vwvayr3g8qhd84le"
+NPUB_C="npub1cld9yay0u24davpu6c35l4vldrhzvaq66pcqtg9a0j2cnjrn9rtsxx2pe6"
+NPUB_D="npub1n9lpnv0592cc2ps6nm0ca3qls642vx7yjsv35rkxqzj2vgds52sqgpverl"
+NPUB_E="npub1wf8akf8lu2zdkjkmwhl75pqvven654mpv4sz2x2tprl5265mgrzq8nhak4"
+
+ping_test() {
+    local from="$1"
+    local to_npub="$2"
+    local label="$3"
+
+    echo -n "  $label ... "
+    if docker exec "fips-$from" ping6 -c "$COUNT" -W "$TIMEOUT" "${to_npub}.fips" > /dev/null 2>&1; then
+        echo "OK"
+        PASSED=$((PASSED + 1))
+    else
+        echo "FAIL"
+        FAILED=$((FAILED + 1))
+    fi
+}
+
+echo "=== FIPS Ping Test ($PROFILE topology) ==="
+echo ""
+
+# Wait for nodes to converge
+echo "Waiting 5s for mesh convergence..."
+sleep 5
+
+if [ "$PROFILE" = "mesh" ]; then
+    # Sparse mesh topology: A-B, B-C, C-D, D-E, E-A, A-D
+    # Test all 20 directed pairs (5 nodes × 4 targets each)
+    echo ""
+    echo "From node-a:"
+    ping_test node-a "$NPUB_B" "A → B"
+    ping_test node-a "$NPUB_C" "A → C"
+    ping_test node-a "$NPUB_D" "A → D"
+    ping_test node-a "$NPUB_E" "A → E"
+
+    echo ""
+    echo "From node-b:"
+    ping_test node-b "$NPUB_A" "B → A"
+    ping_test node-b "$NPUB_C" "B → C"
+    ping_test node-b "$NPUB_D" "B → D"
+    ping_test node-b "$NPUB_E" "B → E"
+
+    echo ""
+    echo "From node-c:"
+    ping_test node-c "$NPUB_A" "C → A"
+    ping_test node-c "$NPUB_B" "C → B"
+    ping_test node-c "$NPUB_D" "C → D"
+    ping_test node-c "$NPUB_E" "C → E"
+
+    echo ""
+    echo "From node-d:"
+    ping_test node-d "$NPUB_A" "D → A"
+    ping_test node-d "$NPUB_B" "D → B"
+    ping_test node-d "$NPUB_C" "D → C"
+    ping_test node-d "$NPUB_E" "D → E"
+
+    echo ""
+    echo "From node-e:"
+    ping_test node-e "$NPUB_A" "E → A"
+    ping_test node-e "$NPUB_B" "E → B"
+    ping_test node-e "$NPUB_C" "E → C"
+    ping_test node-e "$NPUB_D" "E → D"
+
+elif [ "$PROFILE" = "chain" ]; then
+    echo ""
+    echo "Adjacent peer tests:"
+    ping_test node-a "$NPUB_B" "A → B (1 hop)"
+    ping_test node-b "$NPUB_C" "B → C (1 hop)"
+
+    echo ""
+    echo "Multi-hop tests:"
+    ping_test node-a "$NPUB_C" "A → C (2 hops)"
+    ping_test node-a "$NPUB_D" "A → D (3 hops)"
+    ping_test node-a "$NPUB_E" "A → E (4 hops)"
+
+    echo ""
+    echo "Reverse multi-hop:"
+    ping_test node-e "$NPUB_A" "E → A (4 hops)"
+fi
+
+echo ""
+echo "=== Results: $PASSED passed, $FAILED failed ==="
+[ "$FAILED" -eq 0 ] && exit 0 || exit 1