announce server hostname in MANIFEST, display in client

Server reads its hostname via gethostname(2) and includes it as a
length-prefixed UTF-8 string at the start of the MANIFEST payload.

Client displays 'Server: <hostname> — <MAC>' in a label above the
upstream list. Both sides updated for the new MANIFEST format:
  [hostname_len:1][hostname:N][entries...]

Protocol version unchanged (still 1); MANIFEST payload layout change
is backward-incompatible but both sides ship together.

gatunad/src/main.rs

@@ -52,6 +52,8 @@ async fn main() -> ExitCode {
         }
     };
 
+    let hostname = Arc::new(upstream::get_hostname());
+
     let link = match Link::open(&args.iface) {
         Ok(l) => Arc::new(l),
         Err(e) => {
@@ -112,7 +114,7 @@ async fn main() -> ExitCode {
                         continue;
                     }
                 };
-                handle_frame(frame, src, &tx, &store, &next_id, &table).await;
+                handle_frame(frame, src, &tx, &store, &next_id, &table, &hostname).await;
             }
             Ok(None) => {
                 // Ignorable frame (outgoing/short/mismatch) or transient; do not
@@ -135,10 +137,14 @@ async fn handle_frame(
     store: &SessionStore,
     next_id: &Arc<AtomicU32>,
     table: &Arc<crate::upstream::UpstreamTable>,
+    hostname: &Arc<String>,
 ) {
     match frame {
         Frame::Discover => {
-            let manifest = Frame::Manifest(table.entries());
+            let manifest = Frame::Manifest {
+                hostname: (**hostname).clone(),
+                entries: table.entries(),
+            };
             let _ = tx.send((src, manifest.encode())).await;
         }
         Frame::Open { upstream_id } => {
@@ -202,6 +208,6 @@ async fn handle_frame(
             store.lock().expect("store poisoned").remove(&session_id);
         }
         // Not expected from a client; ignore.
-        Frame::Manifest(_) | Frame::OpenAck { .. } | Frame::OpenNak { .. } => {}
+        Frame::Manifest { .. } | Frame::OpenAck { .. } | Frame::OpenNak { .. } => {}
     }
 }