qdisc: expose traffic shaping info via netcfg and LinkDevice trait

LinkDevice gains qdisc_info() → String defaulting to 'none'.
EthernetLink reports current qdisc configuration:
  none  /  token_bucket rate=N burst=N tokens=N  /  priority_queue len=N max=N

TokenBucket and PriorityQueue gain public accessor methods:
  rate(), burst(), tokens() / max_len()

netcfg exposes at /scheme/netcfg/ifaces/eth0/qdisc:
  cat /scheme/netcfg/ifaces/eth0/qdisc  →  'none' (default)

Enables monitoring tools to discover current traffic shaping.
Future: wo node for configuring qdisc type + parameters.

netstack/src/scheme/netcfg/mod.rs

@@ -458,6 +458,14 @@ fn mk_root_node(
                             None => "Device not found\n".into(),
                         }
                     }
+                },
+                "qdisc" => {
+                    ro [devices] || {
+                        match devices.borrow().get("eth0") {
+                            Some(dev) => dev.qdisc_info(),
+                            None => "Device not found\n".into(),
+                        }
+                    }
                 }
             },
             "lo" => {