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/server/slow-backend.py

@@ -0,0 +1,22 @@
+#!/usr/bin/env python3
+# SPDX-License-Identifier: Apache-2.0
+# Minimal HTTP server that sleeps 50 ms before responding.
+# Used by the test harness to produce measurable request durations.
+
+import http.server
+import socketserver
+import time
+
+class SlowHandler(http.server.BaseHTTPRequestHandler):
+    def do_GET(self):
+        time.sleep(0.05)
+        self.send_response(200)
+        self.send_header("Content-Type", "text/plain")
+        self.end_headers()
+        self.wfile.write(b"slow\n")
+
+    def log_message(self, format, *args):
+        pass
+
+with socketserver.TCPServer(("127.0.0.1", 29080), SlowHandler) as srv:
+    srv.serve_forever()