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/link/ethernet.rs

@@ -87,6 +87,10 @@ pub struct EthernetLink {
     ip_address: Option<IpCidr>,
     qdisc_config: QdiscConfig,
     stats: Stats,
+    arp_requests: u64,
+    arp_replies: u64,
+    arp_cache_hits: u64,
+    arp_cache_misses: u64,
 }
 
 impl EthernetLink {
@@ -119,6 +123,10 @@ impl EthernetLink {
             qdisc_config: QdiscConfig::default(),
             stats: Stats::default(),
             neighbor_cache: Default::default(),
+            arp_requests: 0,
+            arp_replies: 0,
+            arp_cache_hits: 0,
+            arp_cache_misses: 0,
         }
     }
 
@@ -205,6 +213,7 @@ impl EthernetLink {
                         expires_at: now + Self::NEIGHBOR_LIVE_TIME,
                     },
                 );
+                self.arp_replies += 1;
 
                 if let ArpOperation::Request = operation {
                     let response = ArpRepr::EthernetIpv4 {
@@ -398,6 +407,7 @@ impl EthernetLink {
     }
 
     fn send_arp(&mut self, now: Instant) {
+        self.arp_requests += 1;
         let Some(hardware_address) = self.hardware_address else {
             return;
         };
@@ -721,12 +731,17 @@ impl LinkDevice for EthernetLink {
         }
 
         match self.neighbor_cache.entry(next_hop) {
-            Entry::Vacant(_) => self.handle_missing_neighbor(next_hop, &packet, now),
+            Entry::Vacant(_) => {
+                self.arp_cache_misses += 1;
+                self.handle_missing_neighbor(next_hop, &packet, now)
+            }
             Entry::Occupied(e) => {
                 if e.get().expires_at < now {
                     e.remove();
+                    self.arp_cache_misses += 1;
                     self.handle_missing_neighbor(next_hop, &packet, now)
                 } else {
+                    self.arp_cache_hits += 1;
                     let mac = e.get().hardware_address;
                     self.send_to(
                         mac,
@@ -849,6 +864,17 @@ impl LinkDevice for EthernetLink {
         self.neighbor_cache.clear();
     }
 
+    fn arp_stats(&self) -> String {
+        format!(
+            "requests={} replies={} hits={} misses={} entries={}\n",
+            self.arp_requests,
+            self.arp_replies,
+            self.arp_cache_hits,
+            self.arp_cache_misses,
+            self.neighbor_cache.len(),
+        )
+    }
+
     fn statistics(&self) -> Stats {
         self.stats
     }

netstack/src/link/mod.rs

@@ -45,6 +45,10 @@ pub trait LinkDevice {
 
     fn flush_arp(&mut self) {}
 
+    fn arp_stats(&self) -> String {
+        String::new()
+    }
+
     fn mtu(&self) -> usize {
         1500
     }

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| {