Client: instant SIGWINCH-driven terminal resize (mosh-style)

The client only polled crossterm size() every 250ms, so window resizes
(esp. in VSCode/terminals that signal rather than expecting polling) lagged
or felt unresponsive. Add a SIGWINCH select branch that resizes the instant
the signal fires, matching mosh (which reacts to SIGWINCH + TIOCGWINSZ). Keep
the 250ms poll as a fallback. Client-only; wire VERSION unchanged.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/bin/dosh-client.rs

@@ -40,6 +40,7 @@ use std::sync::atomic::{AtomicU64, Ordering};
 use std::time::{Duration, Instant, SystemTime, UNIX_EPOCH};
 use tokio::io::{AsyncReadExt, AsyncWriteExt};
 use tokio::net::{TcpListener, TcpStream, UdpSocket, UnixStream};
+use tokio::signal::unix::{SignalKind, signal};
 use tokio::sync::mpsc;
 
 const STREAM_INITIAL_WINDOW: usize = 1024 * 1024;
@@ -2137,6 +2138,11 @@ async fn run_terminal(
     let mut status_tick = tokio::time::interval(Duration::from_secs(1));
     let mut resize_tick = tokio::time::interval(Duration::from_millis(250));
     let mut last_size = terminal_size();
+    // React to terminal resize the instant it happens via SIGWINCH (mosh-style),
+    // instead of waiting up to one `resize_tick`. The 250ms poll below stays as a
+    // fallback for environments where the signal doesn't fire. `signal()` can fail
+    // (rare), in which case we rely on the poll.
+    let mut winch = signal(SignalKind::window_change()).ok();
     let mut frame_buffer = FrameBuffer::default();
     // Resolve the prediction display policy (off / experimental / always). An
     // env var wins for ad-hoc tuning; otherwise the client config's
@@ -2231,17 +2237,16 @@ async fn run_terminal(
                     None => break,
                 }
             }
+            _ = async {
+                match winch.as_mut() {
+                    Some(w) => { w.recv().await; }
+                    None => std::future::pending::<()>().await,
+                }
+            } => {
+                maybe_send_resize(&socket, addr, &cred, &mut send_seq, &mut last_size).await?;
+            }
             _ = resize_tick.tick() => {
-                if let Ok((cols, rows)) = size()
-                    && cols > 0
-                    && rows > 0
-                    && (cols, rows) != last_size
-                {
-                    last_size = (cols, rows);
-                    if cred.mode != "view-only" && cred.mode != "forward-only" {
-                        send_resize(&socket, addr, &cred, &mut send_seq, cols, rows).await?;
-                    }
-                }
+                maybe_send_resize(&socket, addr, &cred, &mut send_seq, &mut last_size).await?;
             }
             recv = socket.recv_from(&mut recv_buf) => {
                 let (n, _) = recv?;
@@ -3713,6 +3718,28 @@ impl FrameBuffer {
     }
 }
 
+/// Read the current terminal size and, if it changed since `last_size`, send a
+/// resize to the server. Shared by the SIGWINCH branch and the poll fallback.
+async fn maybe_send_resize(
+    socket: &UdpSocket,
+    addr: SocketAddr,
+    cred: &CachedCredential,
+    send_seq: &mut u64,
+    last_size: &mut (u16, u16),
+) -> Result<()> {
+    if let Ok((cols, rows)) = size()
+        && cols > 0
+        && rows > 0
+        && (cols, rows) != *last_size
+    {
+        *last_size = (cols, rows);
+        if cred.mode != "view-only" && cred.mode != "forward-only" {
+            send_resize(socket, addr, cred, send_seq, cols, rows).await?;
+        }
+    }
+    Ok(())
+}
+
 async fn send_resize(
     socket: &UdpSocket,
     addr: SocketAddr,