stats: actually track rx_dropped on neighbor-resolution failures

The new rx_dropped counter is now incremented in real drop paths:
- drop_waiting_packets_v4: ARP/NDP failed after MAX_TRIES, packets dropped
- drop_waiting_packets_v6: same for IPv6 neighbor discovery
- handle_missing_neighbor: waiting queue full, incoming packet dropped

Net effect: stats now reflect actual packet loss conditions visible
via /scheme/netcfg/ifaces/eth0/stats:
  rx_dropped=N  (was always 0 before)

tx_dropped was already incremented by qdisc drops (R24).

netstack/src/link/ethernet.rs

@@ -304,6 +304,7 @@ impl EthernetLink {
             }
 
             let _ = self.waiting_packets.dequeue();
+            self.stats.rx_dropped += 1;
             debug!(
                 "Dropped packet on {} because neighbor was not found",
                 self.name
@@ -363,6 +364,7 @@ impl EthernetLink {
                 }
                 Ok((IpAddress::Ipv4(_), _)) => {
                     let _ = self.waiting_packets.dequeue();
+                    self.stats.rx_dropped += 1;
                     continue;
                 }
                 Err(_) => {
@@ -372,12 +374,14 @@ impl EthernetLink {
             }
 
             let _ = self.waiting_packets.dequeue();
+            self.stats.rx_dropped += 1;
             debug!("Dropped packet on {} because IPv6 neighbor was not found", self.name);
         }
     }
 
     fn handle_missing_neighbor(&mut self, next_hop: IpAddress, packet: &[u8], now: Instant) {
         let Ok(buf) = self.waiting_packets.enqueue(packet.len(), next_hop) else {
+            self.stats.rx_dropped += 1;
             warn!(
                 "Dropped packet on {} because waiting queue was full",
                 self.name