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.

win/gatuna-client/SessionManager.cs

@@ -12,6 +12,7 @@ sealed class SessionManager : IDisposable
     readonly ConcurrentDictionary<int, ListenerState> _listeners = new();
 
     byte[]? _serverMac;
+    string _serverHostname = "";
     UpstreamEntry[] _upstreams = [];
 
     // Serialized OPEN: only one outstanding at a time.
@@ -20,10 +21,11 @@ sealed class SessionManager : IDisposable
     readonly Queue<PendingOpen> _openQueue = new();
 
     public event Action<string>? Log;
-    public event Action<UpstreamEntry[]>? ManifestReceived;
+    public event Action<string, byte[], UpstreamEntry[]>? ManifestReceived;
 
     public UpstreamEntry[] Upstreams => _upstreams;
     public byte[]? ServerMac => _serverMac;
+    public string ServerHostname => _serverHostname;
     public TunnelLink? Link => _link;
 
     public void AttachLink(TunnelLink link)
@@ -51,9 +53,10 @@ sealed class SessionManager : IDisposable
         {
             case Frame.Manifest manifest:
                 _serverMac = srcMac;
+                _serverHostname = manifest.Hostname;
                 _upstreams = manifest.Entries;
-                Log?.Invoke($"manifest: {manifest.Entries.Length} upstreams from {BitConverter.ToString(srcMac)}");
-                ManifestReceived?.Invoke(manifest.Entries);
+                Log?.Invoke($"manifest: {manifest.Entries.Length} upstreams from {manifest.Hostname} ({BitConverter.ToString(srcMac)})");
+                ManifestReceived?.Invoke(manifest.Hostname, srcMac, manifest.Entries);
                 break;
 
             case Frame.OpenAck ack: