Implement reactive MtuExceeded error signal (0x22)

Add a new session-layer error signal that transit routers send back to
the source when a forwarded packet exceeds the next-hop transport MTU.
This complements the existing proactive path MTU discovery (min'd at
each hop) by providing immediate feedback when oversized packets are
dropped, closing the transient window before the proactive mechanism
converges.

Wire format: 36-byte payload (msg_type + flags + dest_addr + reporter +
mtu) with FSP phase=0x0 and U flag set, matching the existing
CoordsRequired/PathBroken pattern.

Changes:
- Add SessionMessageType::MtuExceeded (0x22) and MtuExceeded struct with
  encode/decode methods to protocol/session.rs
- Add NodeError::MtuExceeded variant to propagate structured MTU info
  from TransportError through send_encrypted_link_message()
- Catch MtuExceeded in the forwarding path and send error signal back to
  the datagram source via send_mtu_exceeded_error(), rate-limited by the
  existing routing_error_rate_limiter
- Handle incoming MtuExceeded at the source by calling
  PathMtuState::apply_notification() for immediate MTU decrease
- Add unit tests for encode/decode roundtrip, boundary MTU values, and
  too-short payload rejection

src/node/mod.rs

@@ -25,7 +25,7 @@ use crate::peer::{ActivePeer, PeerConnection};
 use self::rate_limit::HandshakeRateLimiter;
 use self::routing_error_rate_limit::RoutingErrorRateLimiter;
 use crate::transport::{
-    Link, LinkId, PacketRx, PacketTx, TransportAddr, TransportHandle, TransportId,
+    Link, LinkId, PacketRx, PacketTx, TransportAddr, TransportError, TransportHandle, TransportId,
 };
 use crate::transport::udp::UdpTransport;
 use crate::tree::TreeState;
@@ -96,6 +96,9 @@ pub enum NodeError {
     #[error("send failed to {node_addr}: {reason}")]
     SendFailed { node_addr: NodeAddr, reason: String },
 
+    #[error("mtu exceeded forwarding to {node_addr}: packet {packet_size} > mtu {mtu}")]
+    MtuExceeded { node_addr: NodeAddr, packet_size: usize, mtu: u16 },
+
     #[error("config error: {0}")]
     Config(#[from] ConfigError),
 
@@ -1156,9 +1159,16 @@ impl Node {
             .ok_or(NodeError::TransportNotFound(transport_id))?;
 
         let bytes_sent = transport.send(&remote_addr, &wire_packet).await
-            .map_err(|e| NodeError::SendFailed {
-                node_addr: *node_addr,
-                reason: format!("transport send: {}", e),
+            .map_err(|e| match e {
+                TransportError::MtuExceeded { packet_size, mtu } => NodeError::MtuExceeded {
+                    node_addr: *node_addr,
+                    packet_size,
+                    mtu,
+                },
+                other => NodeError::SendFailed {
+                    node_addr: *node_addr,
+                    reason: format!("transport send: {}", other),
+                },
             })?;
 
         // Update send statistics