feat: Upgrades networking to use io_uring. (#2315)

> [!IMPORTANT]
> **Breaking Changes**
> - DecodePacket and EncodePacket delegates replaced with IClientEncryption interface
> - NetState.Connection (Socket) replaced with internal RingSocket management
> - NetState.RecvPipe and NetState.SendPipe removed (buffers managed internally)

## Summary

Upgrades the networking stack from PollGroup-based I/O to io_uring, significantly improving I/O performance on Linux.
This also adds native client encryption support for encrypted UO clients.

## Major Changes

io_uring Networking Architecture
- Replaced PollGroup with IORingGroup for async socket I/O operations
- Removed Pipe.cs (mirrored ring buffer) and TcpServer.cs in favor of RingSocketManager
- Added NetState.Network.cs - centralized network infrastructure handling accept, recv, send, and disconnect
completions
- Added SocketHelper.cs - platform-specific socket utilities for raw socket handle operations (getpeername,
getsockname)
- Buffer management now handled by RingSocketManager with configurable slab allocation

### Client Encryption Support
- Added full encryption stack in Network/Encryption/:
  - EncryptionConfig.cs - configurable encryption modes (None, Unencrypted, Encrypted, Both)
  - EncryptionManager.cs - encryption detection and initialization for login/game packets
  - LoginEncryption.cs - handles login packet encryption with version-derived keys
  - GameEncryption.cs - handles game server encryption using Twofish
  - TwofishEngine.cs - optimized Twofish block cipher implementation
  - LoginKeys.cs - encryption key table for client versions
  - IClientEncryption.cs - interface for client encryption implementations

### NetState Improvements
- Replaced Socket Connection with RingSocket _socket for managed socket lifecycle
- Changed from GCHandle polling to event-based completion processing
- Disconnect handling now properly waits for pending sends to flush
- Simplified connecting socket management using lazy queue removal

### Configuration
- New settings: network.encryptionMode and network.encryptionDebug
- Encryption mode flags: Unencrypted, Encrypted, or Both

### Dependencies
- Replaced PollGroup NuGet package with IORingGroup
- Linux requires liburing-dev / liburing-devel package

### Test plan

- Verify server starts and accepts connections on Linux with io_uring
- Verify server starts and accepts connections on Windows (fallback to IOCP)
- Test unencrypted client connections (ClassicUO with encryption disabled)
- Test encrypted client connections if available
- Verify graceful disconnect flushes pending data
- Confirm CI builds pass on all target platforms

Projects/Server.Tests/Tests/Maps/ClientEnumeratorTests.cs

@@ -1,8 +1,8 @@
 using System;
 using System.Collections.Generic;
-using System.Net.Sockets;
 using Server.Accounting;
 using Server.Network;
+using Server.Tests.Network;
 using Xunit;
 
 namespace Server.Tests.Maps;
@@ -356,7 +356,7 @@ public class ClientEnumeratorTests
     {
         var map = Map.Felucca;
         var center = new Point3D(900, 900, 0);
-        var range = 5;
+        const int range = 5;
 
         var clients = new (NetState, Mobile)[3];
         try
@@ -456,8 +456,8 @@ public class ClientEnumeratorTests
 
     private static (NetState, Mobile) CreateClientWithMobile(Map map, Point3D location)
     {
-        var socket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
-        var ns = new NetState(socket);
+        // Create test NetState with real socket and buffers
+        var ns = PacketTestUtilities.CreateTestNetState();
 
         // Assign a mock account to avoid null reference issues
         ns.Account = new MockAccount();
@@ -533,16 +533,16 @@ public class ClientEnumeratorTests
         }
     }
 
-    private static void DeleteAll((NetState, Mobile)[] clients)
+    private static void DeleteAll((NetState state, Mobile m)[] clients)
     {
         for (var i = 0; i < clients.Length; i++)
         {
-            if (clients[i].Item1 != null)
+            if (clients[i].state != null)
             {
-                clients[i].Item1.Mobile = null;
-                clients[i].Item1.Disconnect("Test cleanup");
+                clients[i].state.Mobile = null;
+                clients[i].state.Dispose();
             }
-            clients[i].Item2?.Delete();
+            clients[i].m?.Delete();
         }
     }
 }