Set UDP socket buffer sizes to prevent receive overflow

Under high-throughput forwarding, the kernel default 212KB receive
buffer overflows, silently dropping ~11% of UDP datagrams at transit
nodes. Add configurable recv_buf_size and send_buf_size to UdpConfig
(default 2MB each) using socket2 for pre-bind buffer configuration.
Startup log now reports actual buffer sizes granted by the kernel.

Requires net.core.rmem_max >= 2097152 on the host for the full 2MB
to take effect; otherwise the kernel silently clamps.

src/node/tests/spanning_tree.rs

@@ -25,6 +25,7 @@ pub(super) async fn make_test_node() -> TestNode {
     let udp_config = UdpConfig {
         bind_addr: Some("127.0.0.1:0".to_string()),
         mtu: Some(1280),
+        ..Default::default()
     };
 
     let (packet_tx, packet_rx) = packet_channel(256);