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

tests/01-module/lab/ipng-stats.clab.yml

@@ -0,0 +1,54 @@
+# SPDX-License-Identifier: Apache-2.0
+# Containerlab topology for nginx-ipng-stats-plugin end-to-end tests.
+#
+# Three nodes:
+#   server  — nginx with the module, a slow Python backend, two data-plane interfaces
+#   client1 — sends traffic via eth1 (attributed to source_tag=cl1)
+#   client2 — sends traffic via eth2 (attributed to source_tag=cl2)
+#
+# Links:
+#   server:eth1 ←→ client1:eth1   (10.0.1.0/24)
+#   server:eth2 ←→ client2:eth1   (10.0.2.0/24)
+
+name: ipng-stats-test
+
+mgmt:
+  network: ipng-stats-test-net
+  ipv4-subnet: 172.20.40.0/24
+
+topology:
+  nodes:
+    server:
+      kind: linux
+      image: debian:trixie-slim
+      mgmt-ipv4: 172.20.40.2
+      binds:
+        - ../../../build:/opt/build:ro
+        - ./server/nginx.conf:/opt/config/nginx.conf:ro
+        - ./server/slow-backend.py:/opt/config/slow-backend.py:ro
+        - ./server/start.sh:/start.sh:ro
+      cmd: bash /start.sh
+
+    client1:
+      kind: linux
+      image: debian:trixie-slim
+      mgmt-ipv4: 172.20.40.11
+      binds:
+        - ./client/start.sh:/start.sh:ro
+      cmd: bash /start.sh
+      env:
+        MY_IP: 10.0.1.2/24
+
+    client2:
+      kind: linux
+      image: debian:trixie-slim
+      mgmt-ipv4: 172.20.40.12
+      binds:
+        - ./client/start.sh:/start.sh:ro
+      cmd: bash /start.sh
+      env:
+        MY_IP: 10.0.2.2/24
+
+  links:
+    - endpoints: ["server:eth1", "client1:eth1"]
+    - endpoints: ["server:eth2", "client2:eth1"]