stats: track rx_errors (malformed ARP/frame) + tx_errors (network write fail)

All 4 RFC 1213 counters now wired to real events:
- rx_errors: malformed Ethernet frame or ARP packet
- tx_errors: network_file.write_all() failed
- rx_dropped: ARP/NDP neighbor resolution failed or queue full
- tx_dropped: qdisc returned None (rate-limit kicked in)

Stats output now reflects actual error/drop conditions.
Mirrors Linux's ifconfig/sar 'errors' and 'dropped' fields.

netstack/src/link/ethernet.rs

@@ -156,6 +156,7 @@ impl EthernetLink {
         f(frame.payload_mut());
 
         if let Err(_) = self.network_file.write_all(&self.output_buffer) {
+            self.stats.tx_errors += 1;
             error!(
                 "Dropped outboud packet on {} (failed to write to network file)",
                 self.name
@@ -175,6 +176,7 @@ impl EthernetLink {
         let Ok(repr) = ArpPacket::new_checked(packet).and_then(|packet| ArpRepr::parse(&packet))
         else {
             debug!("Dropped incomming arp packet on {} (Malformed)", self.name);
+            self.stats.rx_errors += 1;
             return;
         };
 
@@ -787,6 +789,7 @@ impl LinkDevice for EthernetLink {
                 let packet = EthernetFrame::new_unchecked(&input_buffer[..]);
                 let Ok(repr) = EthernetRepr::parse(&packet) else {
                     debug!("Dropped incomming frame on {} (Malformed)", self.name);
+                    self.stats.rx_errors += 1;
                     continue;
                 };