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/copyright

@@ -0,0 +1,28 @@
+Format: https://www.debian.org/doc/packaging-manuals/copyright-format/1.0/
+Upstream-Name: nginx-ipng-stats-plugin
+Upstream-Contact: Pim van Pelt <pim@ipng.ch>
+Source: https://git.ipng.ch/ipng/nginx-ipng-stats-plugin
+
+Files: *
+Copyright: 2026 Pim van Pelt <pim@ipng.ch>
+License: Apache-2.0
+
+Files: debian/*
+Copyright: 2026 Pim van Pelt <pim@ipng.ch>
+License: Apache-2.0
+
+License: Apache-2.0
+ Licensed under the Apache License, Version 2.0 (the "License");
+ you may not use this file except in compliance with the License.
+ You may obtain a copy of the License at
+ .
+     http://www.apache.org/licenses/LICENSE-2.0
+ .
+ Unless required by applicable law or agreed to in writing, software
+ distributed under the License is distributed on an "AS IS" BASIS,
+ WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ See the License for the specific language governing permissions and
+ limitations under the License.
+ .
+ On Debian systems, the full text of the Apache License version 2.0
+ can be found in `/usr/share/common-licenses/Apache-2.0'.