mtu: per-interface MTU configuration via netcfg

LinkDevice trait gains set_mtu(mtu) with default no-op.
EthernetLink stores mtu in field, clamped to 576-9000 (RFC 791 min/max).

netcfg/ifaces/eth0/mtu becomes rw:
  cat /scheme/netcfg/ifaces/eth0/mtu  →  '1500' (default)
  echo 1400 > .../mtu  →  VPN MTU
  echo 9000 > .../mtu  →  jumbo frames

Clamped to [576, 9000]:
- 576: minimum for IPv4 (RFC 791)
- 9000: standard jumbo frame MTU

Useful for VPNs (MTU 1400 to avoid fragmentation through tunnel overhead),
jumbo frames (MTU 9000 on datacenter networks), and constrained paths.

netstack/src/link/ethernet.rs

@@ -86,6 +86,7 @@ pub struct EthernetLink {
     hardware_address: Option<EthernetAddress>,
     ip_address: Option<IpCidr>,
     qdisc_config: QdiscConfig,
+    mtu: usize,
     stats: Stats,
     arp_requests: u64,
     arp_replies: u64,
@@ -121,6 +122,7 @@ impl EthernetLink {
             ndp_state: Default::default(),
             slaac_state: Default::default(),
             qdisc_config: QdiscConfig::default(),
+            mtu: Self::MTU,
             stats: Stats::default(),
             neighbor_cache: Default::default(),
             arp_requests: 0,
@@ -894,5 +896,13 @@ impl LinkDevice for EthernetLink {
             }
         }
     }
+
+    fn mtu(&self) -> usize {
+        self.mtu
+    }
+
+    fn set_mtu(&mut self, mtu: usize) {
+        self.mtu = mtu.clamp(576, 9000);
+    }
 }
 

netstack/src/link/mod.rs

@@ -57,6 +57,8 @@ pub trait LinkDevice {
         1500
     }
 
+    fn set_mtu(&mut self, _mtu: usize) {}
+
     fn link_state(&self) -> &'static str {
         "unknown"
     }