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

.gitignore

@@ -0,0 +1,28 @@
+# Build artifacts
+build/
+objs/
+*.so
+*.o
+
+# Debian build outputs (dpkg-buildpackage writes these to the parent dir,
+# but if a helper extracts them here, ignore them)
+*.deb
+*.buildinfo
+*.changes
+debian/.debhelper/
+debian/debhelper-build-stamp
+debian/files
+debian/libnginx-mod-http-ipng-stats/
+debian/tmp/
+debian/*.substvars
+debian/*.log
+
+# Test artifacts
+tests/.venv/
+tests/out/
+
+# Editor/OS cruft
+*.swp
+*.swo
+*~
+.DS_Store