Add Prometheus metrics endpoint; containerize integration tests

Prometheus metrics (internal/metrics/, cmd/maglevd/)
- New --metrics-addr flag (default :9091, env MAGLEV_METRICS_ADDR)
  serving /metrics via promhttp.
- Gauge metrics scraped on demand via a custom prometheus.Collector:
  maglev_backend_state, maglev_backend_health, maglev_backend_enabled,
  maglev_frontend_pool_backend_weight.
- Inline counter/histogram metrics updated per probe:
  maglev_probe_total (by backend, type, result, code),
  maglev_probe_duration_seconds (by backend, type),
  maglev_backend_transitions_total (by backend, from, to).
- StateSource interface in metrics package breaks the import cycle
  with checker; checker.Checker satisfies it via GetBackendInfo.

Integration tests
- Run maglevd inside a containerlab node (debian:trixie-slim with
  build/ bind-mounted) instead of on the host. Eliminates port
  collisions with any host maglevd.
- maglevc commands run via docker exec into the maglevd container.
- Add 6 Prometheus test cases: endpoint reachable, all backends
  report state=up, probe counters non-zero, duration histogram
  populated, pool weights correct, transition counters present.

tests/01-maglevd/maglevd-lab/maglevd.clab.yml

@@ -6,6 +6,15 @@ mgmt:
 
 topology:
   nodes:
+    maglevd:
+      kind: linux
+      image: debian:trixie-slim
+      mgmt-ipv4: 172.20.30.2
+      binds:
+        - ../../../build:/opt/maglev/build:ro
+        - ./maglev.yaml:/etc/maglev/maglev.yaml:ro
+        - ./start.sh:/start.sh:ro
+      cmd: /start.sh
     nginx1:
       kind: linux
       image: nginx:alpine