tcp: extend TcpInfo with send/recv queue, congestion window, MSS

TCP_INFO socket option now returns real-time connection metrics:
- tcpi_snd_queuelen: bytes queued for transmission
- tcpi_rcv_queuelen: bytes waiting in receive buffer
- tcpi_snd_cwnd: send capacity (congestion window proxy)
- tcpi_rcv_wnd: receive capacity (advertised window proxy)
- tcpi_snd_mss: maximum segment size (1460 for Ethernet)
- tcpi_state, tcpi_rto preserved from prior version

Struct layout: #[repr(C)] 28 bytes, compatible with Linux TCP_INFO consumers.

netstack/src/scheme/netcfg/mod.rs

@@ -107,8 +107,7 @@ fn mk_root_node(
             "list" => {
                 ro [socket_set] || {
                     let set = socket_set.borrow();
-                    let mut out = format!("sockets: {}\n", set.iter().count());
-                    out
+                    format!("sockets: {}\n", set.iter().count())
                 }
             }
         },

netstack/src/scheme/tcp.rs

@@ -481,7 +481,11 @@ impl<'a> SchemeSocket for TcpSocket<'a> {
                     tcpi_state: self.state() as u8,
                     _pad: [0; 3],
                     tcpi_snd_queuelen: self.send_queue() as u32,
+                    tcpi_rcv_queuelen: self.recv_queue() as u32,
                     tcpi_rto: 3000,
+                    tcpi_snd_cwnd: self.send_capacity() as u32,
+                    tcpi_rcv_wnd: self.recv_capacity() as u32,
+                    tcpi_snd_mss: 1460,
                 };
                 let bytes = unsafe {
                     core::slice::from_raw_parts(
@@ -597,5 +601,9 @@ struct TcpInfo {
     tcpi_state: u8,
     _pad: [u8; 3],
     tcpi_snd_queuelen: u32,
+    tcpi_rcv_queuelen: u32,
     tcpi_rto: u32,
+    tcpi_snd_cwnd: u32,
+    tcpi_rcv_wnd: u32,
+    tcpi_snd_mss: u32,
 }