feat: remote version query — wzp-client --version-check <relay>

Connects to a relay over QUIC with SNI "version", reads build hash
from a unidirectional stream, prints "<relay> <git-hash>" and exits.

Usage: wzp-client --version-check 172.16.81.175:4434
Output: 172.16.81.175:4434 8dbda3e

Relay side: detects "version" SNI, opens uni stream, writes
BUILD_GIT_HASH, waits 100ms for client to read, closes.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/wzp-client/src/cli.rs

@@ -47,6 +47,7 @@ struct CliArgs {
     room: Option<String>,
     token: Option<String>,
     _metrics_file: Option<String>,
+    version_check: bool,
 }
 
 impl CliArgs {
@@ -88,6 +89,7 @@ fn parse_args() -> CliArgs {
     let mut room = None;
     let mut token = None;
     let mut metrics_file = None;
+    let mut version_check = false;
     let mut relay_str = None;
 
     let mut i = 1;
@@ -169,6 +171,7 @@ fn parse_args() -> CliArgs {
                 );
             }
             "--sweep" => sweep = true,
+            "--version-check" => { version_check = true; }
             "--help" | "-h" => {
                 eprintln!("Usage: wzp-client [options] [relay-addr]");
                 eprintln!();
@@ -221,6 +224,7 @@ fn parse_args() -> CliArgs {
         room,
         token,
         _metrics_file: metrics_file,
+        version_check,
     }
 }
 
@@ -239,6 +243,26 @@ async fn main() -> anyhow::Result<()> {
         return Ok(());
     }
 
+    // --version-check: query relay version over QUIC and exit
+    if cli.version_check {
+        let client_config = wzp_transport::client_config();
+        let bind_addr: SocketAddr = "0.0.0.0:0".parse()?;
+        let endpoint = wzp_transport::create_endpoint(bind_addr, None)?;
+        let conn = wzp_transport::connect(&endpoint, cli.relay_addr, "version", client_config).await?;
+        match conn.accept_uni().await {
+            Ok(mut recv) => {
+                let data = recv.read_to_end(256).await.unwrap_or_default();
+                let version = String::from_utf8_lossy(&data);
+                println!("{} {}", cli.relay_addr, version.trim());
+            }
+            Err(e) => {
+                eprintln!("relay {} does not support version query: {e}", cli.relay_addr);
+            }
+        }
+        endpoint.close(0u32.into(), b"done");
+        return Ok(());
+    }
+
     let seed = cli.resolve_seed();
 
     info!(

crates/wzp-relay/src/main.rs

@@ -490,12 +490,22 @@ async fn main() -> anyhow::Result<()> {
             let transport = Arc::new(wzp_transport::QuinnTransport::new(connection));
 
             // Ping connections: client just measures QUIC connect RTT.
-            // No handshake, no streams — client closes immediately after connecting.
             if room_name == "ping" {
                 info!(%addr, "ping connection (RTT probe)");
                 return;
             }
 
+            // Version query: respond with build hash over a uni stream.
+            if room_name == "version" {
+                if let Ok(mut send) = transport.connection().open_uni().await {
+                    let _ = send.write_all(BUILD_GIT_HASH.as_bytes()).await;
+                    let _ = send.finish();
+                    // Wait for client to read before closing
+                    tokio::time::sleep(std::time::Duration::from_millis(100)).await;
+                }
+                return;
+            }
+
             // Probe connections use SNI "_probe" to identify themselves.
             // They skip auth + handshake and just do Ping->Pong + presence gossip.
             if room_name == "_probe" {