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.prerm

@@ -0,0 +1,26 @@
+#!/bin/sh
+# SPDX-License-Identifier: Apache-2.0
+# prerm for libnginx-mod-http-ipng-stats
+set -e
+
+ENABLED=/etc/nginx/modules-enabled/50-mod-http-ipng-stats.conf
+
+case "$1" in
+    remove|upgrade|deconfigure)
+        if [ -L "$ENABLED" ]; then
+            rm -f "$ENABLED"
+        fi
+        ;;
+
+    failed-upgrade)
+        ;;
+
+    *)
+        echo "prerm called with unknown argument \`$1'" >&2
+        exit 1
+        ;;
+esac
+
+#DEBHELPER#
+
+exit 0