Net: heartbeat liveness, protocol version, message cap, replication reset

Robustness pass over MrGameEng.Net:
- WebSocketServer heartbeats each connection (configurable interval/timeout)
  and drops peers idle past the timeout — detects half-open TCP that vanished
  without a close frame. Tracks last-activity per connection.
- Reassembled messages capped (server and client) so a peer cannot exhaust
  memory with an oversized fragmented message.
- Replication snapshots carry a protocol-version byte; a client receiving a
  mismatched version drops the message instead of decoding garbage, and a
  truncated snapshot is ignored without throwing.
- ReplicationClient.Clear() deletes all replicated entities, so clients can
  wipe stale state before reconnecting.

Tests: heartbeat healthy-survives / silent-peer-dropped, protocol-version
mismatch, truncated snapshot, replication clear.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/MrGameEng.Net.Tests/HeartbeatTests.cs

@@ -0,0 +1,98 @@
+using System.Net;
+using System.Net.Sockets;
+using System.Text;
+using MrGameEng.Net;
+using Xunit;
+
+namespace MrGameEng.Net.Tests;
+
+public class HeartbeatTests
+{
+    [Fact]
+    public async Task HealthyClient_SurvivesPastIdleTimeout()
+    {
+        var port = FreePort();
+        using var server = new WebSocketServer(
+            port,
+            heartbeatInterval: TimeSpan.FromMilliseconds(100),
+            idleTimeout: TimeSpan.FromMilliseconds(400)
+        );
+        server.Start();
+
+        using var client = await WebSocketClient.ConnectAsync(
+            new Uri($"ws://localhost:{port}/"),
+            new CancellationTokenSource(TimeSpan.FromSeconds(10)).Token
+        );
+        var connection = await WaitFor(
+            () => server.TryAcceptConnection(out var c) ? c : null,
+            "server accept"
+        );
+
+        // Дольше idleTimeout: живой клиент авто-отвечает pong на server-ping и остаётся открыт.
+        await Task.Delay(900, TestContext.Current.CancellationToken);
+
+        Assert.True(connection.IsOpen);
+        Assert.True(client.IsOpen);
+    }
+
+    [Fact]
+    public async Task SilentPeer_IsDroppedAfterIdleTimeout()
+    {
+        var port = FreePort();
+        using var server = new WebSocketServer(
+            port,
+            heartbeatInterval: TimeSpan.FromMilliseconds(100),
+            idleTimeout: TimeSpan.FromMilliseconds(400)
+        );
+        server.Start();
+
+        // Сырой peer: проходит рукопожатие, но дальше молчит и не отвечает на ping.
+        using var tcp = new TcpClient();
+        await tcp.ConnectAsync(IPAddress.Loopback, port, TestContext.Current.CancellationToken);
+        var request =
+            "GET / HTTP/1.1\r\n"
+            + "Host: localhost\r\n"
+            + "Upgrade: websocket\r\n"
+            + "Connection: Upgrade\r\n"
+            + "Sec-WebSocket-Key: dGhlIHNhbXBsZSBub25jZQ==\r\n"
+            + "Sec-WebSocket-Version: 13\r\n"
+            + "\r\n";
+        var bytes = Encoding.ASCII.GetBytes(request);
+        await tcp.GetStream().WriteAsync(bytes, TestContext.Current.CancellationToken);
+
+        var connection = await WaitFor(
+            () => server.TryAcceptConnection(out var c) ? c : null,
+            "server accept"
+        );
+        Assert.True(connection.IsOpen);
+
+        // Peer не отвечает pong'ом → активность не обновляется → сервер закрывает по простою.
+        await WaitFor(() => connection.IsOpen ? null : "closed", "idle drop");
+        Assert.False(connection.IsOpen);
+    }
+
+    private static async Task<T> WaitFor<T>(Func<T?> poll, string what)
+        where T : class
+    {
+        for (var i = 0; i < 200; i++)
+        {
+            if (poll() is { } result)
+            {
+                return result;
+            }
+
+            await Task.Delay(25);
+        }
+
+        throw new TimeoutException($"Timed out waiting for {what}.");
+    }
+
+    private static int FreePort()
+    {
+        var listener = new TcpListener(IPAddress.Loopback, 0);
+        listener.Start();
+        var port = ((IPEndPoint)listener.LocalEndpoint).Port;
+        listener.Stop();
+        return port;
+    }
+}