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

config

@@ -0,0 +1,12 @@
+# SPDX-License-Identifier: Apache-2.0
+ngx_addon_name=ngx_http_ipng_stats_module
+
+if test -n "$ngx_module_link"; then
+    ngx_module_type=HTTP
+    ngx_module_name=ngx_http_ipng_stats_module
+    ngx_module_srcs="$ngx_addon_dir/src/ngx_http_ipng_stats_module.c"
+    . auto/module
+else
+    HTTP_MODULES="$HTTP_MODULES ngx_http_ipng_stats_module"
+    NGX_ADDON_SRCS="$NGX_ADDON_SRCS $ngx_addon_dir/src/ngx_http_ipng_stats_module.c"
+fi