stp: enforce blocking on unicast send + enhanced stats display

BridgeDevice STP hardening:
- send(): check STP blocking before unicast forwarding (host-originated)
- recv(): check STP blocking before unicast forwarding (switched frames)
- Previously only flood() checked STP; unicast forwarding bypassed it.
  A blocked port must never forward any traffic — STP semantics now correct.

netcfg stats enhancement:
- Per-interface stats now include mtu= and link= fields alongside counters
- Applies to both eth0 and loopback
- Enables bandwidth monitoring tools to self-discover MTU/link state

netstack/src/link/bridge.rs

@@ -117,6 +117,9 @@ impl LinkDevice for BridgeDevice {
         if repr.dst_addr.is_broadcast() || repr.dst_addr.is_multicast() {
             self.flood(packet, now, None);
         } else if let Some(port_idx) = self.lookup(dst_mac) {
+            if self.stp.borrow().as_ref().is_some_and(|s| s.is_blocked(port_idx)) {
+                return;
+            }
             if let Some(port) = self.ports.borrow_mut().get_mut(port_idx) {
                 port.send(IpAddress::Ipv4(smoltcp::wire::Ipv4Address::UNSPECIFIED), packet, now);
             }
@@ -179,7 +182,9 @@ impl LinkDevice for BridgeDevice {
                     self.flood(&self.recv_buffer, now, Some(port_idx));
                     return Some(&self.recv_buffer);
                 } else if let Some(dst_port_idx) = self.lookup(dst_mac) {
-                    if dst_port_idx != port_idx {
+                    if dst_port_idx != port_idx
+                        && !self.stp.borrow().as_ref().is_some_and(|s| s.is_blocked(dst_port_idx))
+                    {
                         let mut ports = self.ports.borrow_mut();
                         if let Some(target) = ports.get_mut(dst_port_idx) {
                             target.send(IpAddress::Ipv4(smoltcp::wire::Ipv4Address::UNSPECIFIED), &packet, now);