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/ReplicationTests.cs

@@ -103,8 +103,8 @@ public class ReplicationTests
         server.Send([connection]);
 
         Assert.True(connection.Sent.TryDequeue(out var delta));
-        // Запись: type(1) + count(4) + netId(4) + op(1) + mask(4) + TestPosition(8) — без TestHealth.
-        Assert.Equal(22, delta!.Length);
+        // Запись: type(1) + version(1) + count(4) + netId(4) + op(1) + mask(4) + TestPosition(8) — без TestHealth.
+        Assert.Equal(23, delta!.Length);
 
         client.Apply(delta);
         var replicated = FindByNetId(clientStore, 1);
@@ -185,6 +185,81 @@ public class ReplicationTests
         Assert.Equal(1, spawns);
     }
 
+    [Fact]
+    public void Clear_DeletesEveryReplicatedEntity()
+    {
+        var serverStore = new EntityStore();
+        var clientStore = new EntityStore();
+        var server = new ReplicationServer(MakeSchema(), serverStore);
+        var client = new ReplicationClient(MakeSchema(), clientStore);
+        var connection = new FakeConnection();
+        serverStore.CreateEntity(
+            new NetId { Value = server.NextNetId() },
+            new TestPosition { X = 1f, Y = 2f }
+        );
+        serverStore.CreateEntity(
+            new NetId { Value = server.NextNetId() },
+            new TestPosition { X = 3f, Y = 4f }
+        );
+        server.Send([connection]);
+        client.Pump(connection);
+        Assert.Equal(2, client.EntityCount);
+
+        client.Clear();
+
+        Assert.Equal(0, client.EntityCount);
+        foreach (var entity in clientStore.Entities)
+        {
+            Assert.False(entity.HasComponent<NetId>());
+        }
+    }
+
+    [Fact]
+    public void MismatchedProtocolVersion_IsDropped()
+    {
+        var serverStore = new EntityStore();
+        var clientStore = new EntityStore();
+        var server = new ReplicationServer(MakeSchema(), serverStore);
+        var client = new ReplicationClient(MakeSchema(), clientStore);
+        var connection = new FakeConnection();
+        serverStore.CreateEntity(
+            new NetId { Value = server.NextNetId() },
+            new TestPosition { X = 1f, Y = 2f }
+        );
+        server.Send([connection]);
+        Assert.True(connection.Sent.TryDequeue(out var snapshot));
+
+        // Портим байт версии (индекс 1: type=0, version=1) — клиент обязан отбросить снапшот целиком.
+        snapshot![1] = 0xFF;
+        client.Apply(snapshot);
+
+        Assert.Equal(0, client.EntityCount);
+    }
+
+    [Fact]
+    public void TruncatedSnapshot_IsIgnoredWithoutThrowing()
+    {
+        var serverStore = new EntityStore();
+        var clientStore = new EntityStore();
+        var server = new ReplicationServer(MakeSchema(), serverStore);
+        var client = new ReplicationClient(MakeSchema(), clientStore);
+        var connection = new FakeConnection();
+        serverStore.CreateEntity(
+            new NetId { Value = server.NextNetId() },
+            new TestPosition { X = 5f, Y = 6f },
+            new TestHealth { Value = 9 }
+        );
+        server.Send([connection]);
+        Assert.True(connection.Sent.TryDequeue(out var snapshot));
+
+        // Режем хвост: заголовок и счётчик целы, но данные компонентов оборваны.
+        var truncated = snapshot!.AsSpan(0, snapshot.Length - 6).ToArray();
+        client.Apply(truncated); // не должно бросить
+
+        // Записи могли частично примениться, но клиент остался живым и консистентным.
+        Assert.True(client.EntityCount <= 1);
+    }
+
     private static Entity FindByNetId(EntityStore store, int netId)
     {
         foreach (var entity in store.Entities)