Add per-link MTU support to Transport trait

Add link_mtu(&TransportAddr) method to the Transport trait with a
default implementation that falls back to the transport-wide mtu().
This enables transports like BLE to report per-connection MTU values
while maintaining backward compatibility for UDP and other transports
that use a single MTU for all links.

Update the forwarding and session send paths to query link_mtu() with
the next-hop peer's current address, falling back to transport-wide
mtu() when no address is available.

src/node/handlers/forwarding.rs

@@ -64,7 +64,11 @@ impl Node {
             && let Some(tid) = peer.transport_id()
             && let Some(transport) = self.transports.get(&tid)
         {
-            datagram.path_mtu = datagram.path_mtu.min(transport.mtu());
+            if let Some(addr) = peer.current_addr() {
+                datagram.path_mtu = datagram.path_mtu.min(transport.link_mtu(addr));
+            } else {
+                datagram.path_mtu = datagram.path_mtu.min(transport.mtu());
+            }
         }
 
         // Forward: re-encode (includes 0x00 type byte) and send

src/node/handlers/session.rs

@@ -1062,7 +1062,11 @@ impl Node {
             && let Some(tid) = peer.transport_id()
             && let Some(transport) = self.transports.get(&tid)
         {
-            datagram.path_mtu = datagram.path_mtu.min(transport.mtu());
+            if let Some(addr) = peer.current_addr() {
+                datagram.path_mtu = datagram.path_mtu.min(transport.link_mtu(addr));
+            } else {
+                datagram.path_mtu = datagram.path_mtu.min(transport.mtu());
+            }
         }
 
         // Source-side: seed our PathMtuState.current_mtu from the outbound

src/transport/mod.rs

@@ -740,6 +740,16 @@ pub trait Transport {
     /// Get the MTU for this transport.
     fn mtu(&self) -> u16;
 
+    /// Get the MTU for a specific link.
+    ///
+    /// Returns the MTU negotiated for the given transport address, or
+    /// falls back to the transport-wide default if the address is unknown
+    /// or the transport doesn't support per-link MTU negotiation.
+    fn link_mtu(&self, addr: &TransportAddr) -> u16 {
+        let _ = addr;
+        self.mtu()
+    }
+
     /// Start the transport.
     fn start(&mut self) -> Result<(), TransportError>;
 
@@ -824,6 +834,16 @@ impl TransportHandle {
         }
     }
 
+    /// Get the MTU for a specific link address.
+    ///
+    /// Falls back to transport-wide MTU if the transport doesn't
+    /// support per-link MTU or the address is unknown.
+    pub fn link_mtu(&self, addr: &TransportAddr) -> u16 {
+        match self {
+            TransportHandle::Udp(t) => t.link_mtu(addr),
+        }
+    }
+
     /// Get the local bound address (only valid after start).
     pub fn local_addr(&self) -> Option<std::net::SocketAddr> {
         match self {
@@ -1116,4 +1136,156 @@ mod tests {
         let received = rx.recv().await.unwrap();
         assert_eq!(received.data, vec![1, 2, 3]);
     }
+
+    // ========================================================================
+    // link_mtu tests
+    // ========================================================================
+
+    /// Minimal mock transport for testing the default link_mtu() behavior.
+    struct MockTransport {
+        id: TransportId,
+        mtu_value: u16,
+    }
+
+    impl MockTransport {
+        fn new(mtu: u16) -> Self {
+            Self {
+                id: TransportId::new(99),
+                mtu_value: mtu,
+            }
+        }
+    }
+
+    impl Transport for MockTransport {
+        fn transport_id(&self) -> TransportId {
+            self.id
+        }
+        fn transport_type(&self) -> &TransportType {
+            &TransportType::UDP
+        }
+        fn state(&self) -> TransportState {
+            TransportState::Up
+        }
+        fn mtu(&self) -> u16 {
+            self.mtu_value
+        }
+        fn start(&mut self) -> Result<(), TransportError> {
+            Ok(())
+        }
+        fn stop(&mut self) -> Result<(), TransportError> {
+            Ok(())
+        }
+        fn send(&self, _addr: &TransportAddr, _data: &[u8]) -> Result<(), TransportError> {
+            Ok(())
+        }
+        fn discover(&self) -> Result<Vec<DiscoveredPeer>, TransportError> {
+            Ok(vec![])
+        }
+    }
+
+    /// Mock transport that overrides link_mtu() to return per-link values.
+    struct PerLinkMtuTransport {
+        id: TransportId,
+        default_mtu: u16,
+        /// Address-specific MTU overrides.
+        overrides: Vec<(TransportAddr, u16)>,
+    }
+
+    impl PerLinkMtuTransport {
+        fn new(default_mtu: u16, overrides: Vec<(TransportAddr, u16)>) -> Self {
+            Self {
+                id: TransportId::new(100),
+                default_mtu,
+                overrides,
+            }
+        }
+    }
+
+    impl Transport for PerLinkMtuTransport {
+        fn transport_id(&self) -> TransportId {
+            self.id
+        }
+        fn transport_type(&self) -> &TransportType {
+            &TransportType::UDP
+        }
+        fn state(&self) -> TransportState {
+            TransportState::Up
+        }
+        fn mtu(&self) -> u16 {
+            self.default_mtu
+        }
+        fn link_mtu(&self, addr: &TransportAddr) -> u16 {
+            for (a, mtu) in &self.overrides {
+                if a == addr {
+                    return *mtu;
+                }
+            }
+            self.mtu()
+        }
+        fn start(&mut self) -> Result<(), TransportError> {
+            Ok(())
+        }
+        fn stop(&mut self) -> Result<(), TransportError> {
+            Ok(())
+        }
+        fn send(&self, _addr: &TransportAddr, _data: &[u8]) -> Result<(), TransportError> {
+            Ok(())
+        }
+        fn discover(&self) -> Result<Vec<DiscoveredPeer>, TransportError> {
+            Ok(vec![])
+        }
+    }
+
+    #[test]
+    fn test_link_mtu_default_falls_back_to_mtu() {
+        let transport = MockTransport::new(1280);
+        let addr = TransportAddr::from_string("192.168.1.1:4000");
+
+        // Default link_mtu() should return the transport-wide mtu()
+        assert_eq!(transport.link_mtu(&addr), 1280);
+        assert_eq!(transport.link_mtu(&addr), transport.mtu());
+
+        // Any address should return the same value
+        let other_addr = TransportAddr::from_string("10.0.0.1:5000");
+        assert_eq!(transport.link_mtu(&other_addr), 1280);
+    }
+
+    #[test]
+    fn test_link_mtu_per_link_override() {
+        let addr_a = TransportAddr::from_string("192.168.1.1:4000");
+        let addr_b = TransportAddr::from_string("10.0.0.1:5000");
+        let addr_unknown = TransportAddr::from_string("172.16.0.1:6000");
+
+        let transport = PerLinkMtuTransport::new(
+            1280,
+            vec![(addr_a.clone(), 512), (addr_b.clone(), 247)],
+        );
+
+        // Known addresses return their per-link MTU
+        assert_eq!(transport.link_mtu(&addr_a), 512);
+        assert_eq!(transport.link_mtu(&addr_b), 247);
+
+        // Unknown address falls back to transport-wide default
+        assert_eq!(transport.link_mtu(&addr_unknown), 1280);
+        assert_eq!(transport.mtu(), 1280);
+    }
+
+    #[test]
+    fn test_transport_handle_link_mtu_delegation() {
+        use crate::config::UdpConfig;
+        use crate::transport::udp::UdpTransport;
+
+        let config = UdpConfig::default();
+        let expected_mtu = config.mtu();
+        let (tx, _rx) = packet_channel(1);
+        let transport = UdpTransport::new(TransportId::new(1), None, config, tx);
+        let handle = TransportHandle::Udp(transport);
+
+        let addr = TransportAddr::from_string("192.168.1.1:4000");
+
+        // TransportHandle::link_mtu() should delegate and return the same
+        // as TransportHandle::mtu() for UDP (no per-link overrides)
+        assert_eq!(handle.link_mtu(&addr), expected_mtu);
+        assert_eq!(handle.link_mtu(&addr), handle.mtu());
+    }
 }