stats: add rx_errors, tx_errors, rx_dropped, tx_dropped counters

Stats struct gains 4 new u64 fields per RFC 1213 (MIB-II):
- rx_errors: incoming packets with errors
- tx_errors: outgoing packets with errors
- rx_dropped: incoming packets dropped (buffer full, etc.)
- tx_dropped: outgoing packets dropped (qdisc dropped, buffer full)

EthernetLink tx path now increments tx_dropped when qdisc returns None
(qdisc decided to drop the packet, e.g., TokenBucket rate exceeded).

netcfg stats output now exposes all 8 counters:
  rx_bytes=N rx_packets=N tx_bytes=N tx_packets=N
  rx_errors=N tx_errors=N rx_dropped=N tx_dropped=N
  mtu=N link=up

Aggregate fields work for bridge/bond via existing sum logic.

netstack/src/link/ethernet.rs

@@ -711,6 +711,7 @@ impl LinkDevice for EthernetLink {
 
         let owned = self.qdisc_config.enqueue_and_dequeue(packet.to_vec(), now);
         let Some(packet) = owned else {
+            self.stats.tx_dropped += 1;
             return;
         };
         self.stats.tx_packets += 1;

netstack/src/link/mod.rs

@@ -84,6 +84,10 @@ pub struct Stats {
     pub rx_packets: u64,
     pub tx_bytes: u64,
     pub tx_packets: u64,
+    pub rx_errors: u64,
+    pub tx_errors: u64,
+    pub rx_dropped: u64,
+    pub tx_dropped: u64,
 }
 
 #[derive(Default)]