arp: add statistics counters and netcfg exposure

EthernetLink gains per-interface ARP counters:
- arp_requests: ARP requests sent (incremented in send_arp)
- arp_replies: ARP replies received (incremented on cache insert)
- arp_cache_hits: successful neighbor cache lookups
- arp_cache_misses: cache misses (vacant or expired entries)

LinkDevice trait gains arp_stats() → String method.
EthernetLink implementation: 'requests=N replies=N hits=N misses=N entries=N'

netcfg exposes at /scheme/netcfg/ifaces/eth0/arp/stats:
  echo /scheme/netcfg/ifaces/eth0/arp/stats
  → requests=5 replies=3 hits=142 misses=8 entries=4

Existing arp/list (MAC→IP) and arp/flush (clear) are unchanged.
Useful for monitoring ARP churn, detecting ARP storms, and debugging
neighbor discovery issues.

netstack/src/scheme/netcfg/mod.rs

@@ -360,6 +360,14 @@ fn mk_root_node(
                             }
                         }
                     },
+                    "stats" => {
+                        ro [devices] || {
+                            match devices.borrow().get("eth0") {
+                                Some(dev) => dev.arp_stats(),
+                                None => "Device not found\n".into(),
+                            }
+                        }
+                    },
                     "flush" => {
                         wo [devices] (Option<()>, None)
                         |_cur_value, _line| {