Add ngx_http_ipng_stats_module: per-VIP, per-device traffic counters

Full implementation of the nginx dynamic module with:
- SO_BINDTODEVICE-based per-interface traffic attribution
- Per-worker lock-free counters flushed to shared memory
- Prometheus text and JSON scrape endpoint at configurable location
- UDP-only global logtail (ipng_stats_logtail) for fire-and-forget
  access log streaming
- $ipng_source_tag nginx variable for use in log_format/map
- Histogram buckets, EWMA rate gauges, zone meta-metrics
- Debian packaging (libnginx-mod-http-ipng-stats)
- Robot Framework end-to-end tests via containerlab
- SPDX Apache-2.0 headers on all source files

debian/libnginx-mod-http-ipng-stats.postinst

@@ -0,0 +1,42 @@
+#!/bin/sh
+# SPDX-License-Identifier: Apache-2.0
+# postinst for libnginx-mod-http-ipng-stats
+set -e
+
+AVAIL=/etc/nginx/modules-available/50-mod-http-ipng-stats.conf
+ENABLED=/etc/nginx/modules-enabled/50-mod-http-ipng-stats.conf
+
+case "$1" in
+    configure)
+        # Enable the module by symlinking it into modules-enabled.
+        if [ ! -L "$ENABLED" ] && [ -f "$AVAIL" ]; then
+            ln -s "$AVAIL" "$ENABLED"
+        fi
+
+        # Sanity-check the resulting config. If nginx -t fails, back
+        # out the symlink so the operator isn't left with an nginx
+        # that cannot start.
+        if ! nginx -t > /dev/null 2>&1; then
+            echo "warning: nginx -t failed after enabling" \
+                 "libnginx-mod-http-ipng-stats; disabling the module." >&2
+            rm -f "$ENABLED"
+            nginx -t >&2 || true
+            echo "warning: nginx-ipng-stats-plugin has been installed" \
+                 "but is NOT enabled; fix the configuration error" \
+                 "and re-enable with:" >&2
+            echo "  sudo ln -s $AVAIL $ENABLED && sudo nginx -s reload" >&2
+        fi
+        ;;
+
+    abort-upgrade|abort-remove|abort-deconfigure)
+        ;;
+
+    *)
+        echo "postinst called with unknown argument \`$1'" >&2
+        exit 1
+        ;;
+esac
+
+#DEBHELPER#
+
+exit 0