Add MrGameEng.Net: WebSocket transport + delta component replication

Browsers can't speak UDP, so WebSocket is the engine's one transport.
The server side is a dependency-free RFC 6455 implementation over
TcpListener (handshake, frame codec with masking and fragmentation,
ping/pong — unit-tested against the RFC example vectors); the client
wraps ClientWebSocket, which works on desktop and maps to the browser
WebSocket in Blazor WASM. Both sit behind the poll-based INetConnection
so simulation systems drain messages from their own thread; client
sends are chained fire-and-forget (no blocking — wasm-safe).

Replication is server-authoritative: games register unmanaged component
types in a ReplicationSchema (same order both sides, up to 32 types),
ReplicationServer snapshots entities carrying NetId once per send and
ships each connection only the components that changed since its last
snapshot — a reliable ordered transport needs no acks for deltas. New
connections receive the full state through the same path; despawns are
tracked by set difference. ReplicationClient applies snapshots to a
local EntityStore and raises EntitySpawned so the game can decorate
replicated entities with presentation components.

Covered by 16 tests including a real loopback exchange between
WebSocketClient and WebSocketServer.

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

src/MrGameEng.Net/Replication/NetId.cs

@@ -0,0 +1,14 @@
+using Friflo.Engine.ECS;
+
+namespace MrGameEng.Net;
+
+/// <summary>
+/// Marks an entity as replicated and identifies it across the network. The server assigns
+/// values (see <see cref="ReplicationServer.NextNetId"/>); the client creates a local
+/// entity with the same <see cref="Value"/> when the first snapshot arrives.
+/// </summary>
+public struct NetId : IComponent
+{
+    /// <summary>Network-wide entity id, unique per server world.</summary>
+    public int Value;
+}

src/MrGameEng.Net/Replication/ReplicationClient.cs

@@ -0,0 +1,92 @@
+using Friflo.Engine.ECS;
+
+namespace MrGameEng.Net;
+
+/// <summary>
+/// Client side of replication: applies snapshot messages from a
+/// <see cref="ReplicationServer"/> to a local <see cref="EntityStore"/>. Unknown net ids
+/// spawn local entities (carrying <see cref="NetId"/>), known ones get their changed
+/// components overwritten, despawns delete. The game decorates replicated entities with
+/// presentation components (sprites etc.) on top — replication never touches types outside
+/// its <see cref="ReplicationSchema"/>.
+/// </summary>
+public sealed class ReplicationClient
+{
+    /// <summary>Number of replicated entities currently alive locally.</summary>
+    public int EntityCount => _entities.Count;
+
+    /// <summary>Raised after an entity is created from a snapshot. Hook presentation setup here.</summary>
+    public event Action<Entity>? EntitySpawned;
+
+    private readonly ReplicationSchema _schema;
+    private readonly EntityStore _store;
+    private readonly Dictionary<int, Entity> _entities = [];
+
+    /// <summary>Creates a replication client writing into <paramref name="store"/>.</summary>
+    public ReplicationClient(ReplicationSchema schema, EntityStore store)
+    {
+        _schema = schema;
+        _store = store;
+    }
+
+    /// <summary>Applies every message queued on <paramref name="connection"/>.</summary>
+    public void Pump(INetConnection connection)
+    {
+        while (connection.TryReceive(out var message))
+        {
+            Apply(message);
+        }
+    }
+
+    /// <summary>Applies one snapshot message to the local store.</summary>
+    public void Apply(byte[] message)
+    {
+        using var reader = new BinaryReader(new MemoryStream(message));
+        if (reader.ReadByte() != ReplicationMessage.Snapshot)
+        {
+            return; // незнакомый тип сообщения — пропускаем, это не снапшот
+        }
+
+        var slots = _schema.Slots;
+        var count = reader.ReadInt32();
+        for (var record = 0; record < count; record++)
+        {
+            var netId = reader.ReadInt32();
+            var op = reader.ReadByte();
+            if (op == ReplicationMessage.OpDespawn)
+            {
+                if (_entities.Remove(netId, out var dead))
+                {
+                    dead.DeleteEntity();
+                }
+
+                continue;
+            }
+
+            var mask = reader.ReadUInt32();
+            var spawned = false;
+            if (!_entities.TryGetValue(netId, out var entity))
+            {
+                entity = _store.CreateEntity(new NetId { Value = netId });
+                _entities[netId] = entity;
+                spawned = true;
+            }
+
+            foreach (var slot in slots)
+            {
+                if ((mask & (1u << slot.Bit)) == 0)
+                {
+                    continue;
+                }
+
+                var data = reader.ReadBytes(slot.Size);
+                slot.Apply(entity, data, 0);
+            }
+
+            if (spawned)
+            {
+                EntitySpawned?.Invoke(entity);
+            }
+        }
+    }
+}

src/MrGameEng.Net/Replication/ReplicationSchema.cs

@@ -0,0 +1,64 @@
+using System.Runtime.CompilerServices;
+using System.Runtime.InteropServices;
+using Friflo.Engine.ECS;
+
+namespace MrGameEng.Net;
+
+/// <summary>
+/// The set of component types a game replicates, registered in the same order on the
+/// server and every client (the order defines the wire ids). Components must be
+/// unmanaged structs — they are blitted to the wire as raw bytes, so server and client
+/// must run on the same engine version. Up to 32 types.
+/// </summary>
+public sealed class ReplicationSchema
+{
+    internal sealed class ComponentSlot
+    {
+        public required int Bit;
+        public required int Size;
+        public required Func<Entity, byte[], bool> TryWrite;
+        public required Action<Entity, byte[], int> Apply;
+    }
+
+    internal readonly List<ComponentSlot> Slots = [];
+
+    /// <summary>
+    /// Registers component type <typeparamref name="T"/> for replication. Returns this
+    /// schema for fluent chaining.
+    /// </summary>
+    public ReplicationSchema Register<T>()
+        where T : unmanaged, IComponent
+    {
+        if (Slots.Count == 32)
+        {
+            throw new InvalidOperationException(
+                "ReplicationSchema supports at most 32 component types."
+            );
+        }
+
+        var size = Unsafe.SizeOf<T>();
+        Slots.Add(
+            new ComponentSlot
+            {
+                Bit = Slots.Count,
+                Size = size,
+                TryWrite = (entity, buffer) =>
+                {
+                    if (!entity.HasComponent<T>())
+                    {
+                        return false;
+                    }
+
+                    MemoryMarshal.Write(buffer, in entity.GetComponent<T>());
+                    return true;
+                },
+                Apply = (entity, data, offset) =>
+                {
+                    var value = MemoryMarshal.Read<T>(data.AsSpan(offset, size));
+                    entity.AddComponent(value);
+                },
+            }
+        );
+        return this;
+    }
+}

src/MrGameEng.Net/Replication/ReplicationServer.cs

@@ -0,0 +1,174 @@
+using Friflo.Engine.ECS;
+
+namespace MrGameEng.Net;
+
+/// <summary>
+/// Server-authoritative component replication. Each call to <see cref="Send"/> snapshots
+/// every entity carrying <see cref="NetId"/> and sends each connection only what changed
+/// since that connection's previous snapshot (per-component deltas; a new connection gets
+/// the full state the same way). Deltas need no acknowledgements because the transport is
+/// reliable and ordered. Call at the desired send rate (e.g. every Nth simulation tick),
+/// from the simulation thread.
+/// </summary>
+public sealed class ReplicationServer
+{
+    private sealed class ConnectionState
+    {
+        // По netId: последний отправленный блоб каждого зарегистрированного компонента.
+        public readonly Dictionary<int, byte[]?[]> LastSent = [];
+    }
+
+    private readonly ReplicationSchema _schema;
+    private readonly ArchetypeQuery<NetId> _query;
+    private readonly Dictionary<INetConnection, ConnectionState> _states = [];
+
+    // Снапшот текущего тика, переиспользуется между соединениями.
+    private readonly List<(int NetId, byte[]?[] Components)> _current = [];
+    private readonly HashSet<int> _currentIds = [];
+    private int _nextNetId;
+
+    /// <summary>Creates a replication server over <paramref name="store"/>.</summary>
+    public ReplicationServer(ReplicationSchema schema, EntityStore store)
+    {
+        _schema = schema;
+        _query = store.Query<NetId>();
+    }
+
+    /// <summary>Allocates the next free network id for a newly spawned replicated entity.</summary>
+    public int NextNetId() => ++_nextNetId;
+
+    /// <summary>
+    /// Snapshots the world once and sends per-connection deltas. Closed connections are
+    /// forgotten; brand-new ones receive the full state.
+    /// </summary>
+    public void Send(IReadOnlyList<INetConnection> connections)
+    {
+        CaptureCurrentState();
+
+        foreach (var connection in connections)
+        {
+            if (!connection.IsOpen)
+            {
+                continue;
+            }
+
+            if (!_states.TryGetValue(connection, out var state))
+            {
+                state = new ConnectionState();
+                _states[connection] = state;
+            }
+
+            var message = BuildDelta(state);
+            if (message is not null)
+            {
+                connection.Send(message);
+            }
+        }
+
+        // Забываем состояние умерших соединений, чтобы не копить мусор.
+        foreach (var dead in _states.Keys.Where(c => !c.IsOpen).ToList())
+        {
+            _states.Remove(dead);
+        }
+    }
+
+    private void CaptureCurrentState()
+    {
+        _current.Clear();
+        _currentIds.Clear();
+        var slots = _schema.Slots;
+        _query.ForEachEntity(
+            (ref NetId netId, Entity entity) =>
+            {
+                var components = new byte[]?[slots.Count];
+                foreach (var slot in slots)
+                {
+                    var buffer = new byte[slot.Size];
+                    components[slot.Bit] = slot.TryWrite(entity, buffer) ? buffer : null;
+                }
+
+                _current.Add((netId.Value, components));
+                _currentIds.Add(netId.Value);
+            }
+        );
+    }
+
+    private byte[]? BuildDelta(ConnectionState state)
+    {
+        using var stream = new MemoryStream();
+        using var writer = new BinaryWriter(stream);
+        writer.Write(ReplicationMessage.Snapshot);
+        var countPosition = stream.Position;
+        writer.Write(0); // количество записей, допишем в конце
+        var records = 0;
+
+        foreach (var (netId, components) in _current)
+        {
+            if (!state.LastSent.TryGetValue(netId, out var lastSent))
+            {
+                lastSent = new byte[]?[components.Length];
+                state.LastSent[netId] = lastSent;
+            }
+
+            uint mask = 0;
+            for (var bit = 0; bit < components.Length; bit++)
+            {
+                var current = components[bit];
+                if (current is null)
+                {
+                    continue;
+                }
+
+                if (lastSent[bit] is null || !current.AsSpan().SequenceEqual(lastSent[bit]))
+                {
+                    mask |= 1u << bit;
+                    lastSent[bit] = current;
+                }
+            }
+
+            if (mask == 0)
+            {
+                continue;
+            }
+
+            writer.Write(netId);
+            writer.Write(ReplicationMessage.OpUpsert);
+            writer.Write(mask);
+            for (var bit = 0; bit < components.Length; bit++)
+            {
+                if ((mask & (1u << bit)) != 0)
+                {
+                    writer.Write(components[bit]!);
+                }
+            }
+
+            records++;
+        }
+
+        // Сущности, которые соединение знает, а в мире их больше нет.
+        foreach (var known in state.LastSent.Keys.Where(id => !_currentIds.Contains(id)).ToList())
+        {
+            state.LastSent.Remove(known);
+            writer.Write(known);
+            writer.Write(ReplicationMessage.OpDespawn);
+            records++;
+        }
+
+        if (records == 0)
+        {
+            return null;
+        }
+
+        stream.Position = countPosition;
+        writer.Write(records);
+        return stream.ToArray();
+    }
+}
+
+/// <summary>Wire constants shared by <see cref="ReplicationServer"/> and <see cref="ReplicationClient"/>.</summary>
+internal static class ReplicationMessage
+{
+    internal const byte Snapshot = 1;
+    internal const byte OpUpsert = 0;
+    internal const byte OpDespawn = 1;
+}