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/Network/Packets/Outgoing/AccountPacketTests.cs

@@ -92,7 +92,7 @@ public class AccountPacketTests
         using var ns = PacketTestUtilities.CreateTestNetState();
         ns.SendChangeCharacter(account);
 
-        var result = ns.SendPipe.Reader.AvailableToRead();
+        var result = ns.SendBuffer.GetReadSpan();
         AssertThat.Equal(result, expected);
     }
 
@@ -105,7 +105,7 @@ public class AccountPacketTests
 
         ns.SendClientVersionRequest();
 
-        var result = ns.SendPipe.Reader.AvailableToRead();
+        var result = ns.SendBuffer.GetReadSpan();
         AssertThat.Equal(result, expected);
     }
 
@@ -117,7 +117,7 @@ public class AccountPacketTests
         using var ns = PacketTestUtilities.CreateTestNetState();
         ns.SendCharacterDeleteResult(DeleteResultType.BadRequest);
 
-        var result = ns.SendPipe.Reader.AvailableToRead();
+        var result = ns.SendBuffer.GetReadSpan();
         AssertThat.Equal(result, expected);
     }
 
@@ -129,7 +129,7 @@ public class AccountPacketTests
         using var ns = PacketTestUtilities.CreateTestNetState();
         ns.SendPopupMessage(PMMessage.LoginSyncError);
 
-        var result = ns.SendPipe.Reader.AvailableToRead();
+        var result = ns.SendBuffer.GetReadSpan();
         AssertThat.Equal(result, expected);
     }
 
@@ -156,7 +156,7 @@ public class AccountPacketTests
         var expected = new SupportedFeatures(ns).Compile();
         ns.SendSupportedFeature();
 
-        var result = ns.SendPipe.Reader.AvailableToRead();
+        var result = ns.SendBuffer.GetReadSpan();
         AssertThat.Equal(result, expected);
     }
 
@@ -177,7 +177,7 @@ public class AccountPacketTests
         using var ns = PacketTestUtilities.CreateTestNetState();
         ns.SendLoginConfirmation(m);
 
-        var result = ns.SendPipe.Reader.AvailableToRead();
+        var result = ns.SendBuffer.GetReadSpan();
         AssertThat.Equal(result, expected);
     }
 
@@ -189,7 +189,7 @@ public class AccountPacketTests
         using var ns = PacketTestUtilities.CreateTestNetState();
         ns.SendLoginComplete();
 
-        var result = ns.SendPipe.Reader.AvailableToRead();
+        var result = ns.SendBuffer.GetReadSpan();
         AssertThat.Equal(result, expected);
     }
 
@@ -214,7 +214,7 @@ public class AccountPacketTests
         using var ns = PacketTestUtilities.CreateTestNetState();
         ns.SendCharacterListUpdate(account);
 
-        var result = ns.SendPipe.Reader.AvailableToRead();
+        var result = ns.SendBuffer.GetReadSpan();
         AssertThat.Equal(result, expected);
     }
 
@@ -248,7 +248,7 @@ public class AccountPacketTests
 
         ns.SendCharacterList();
 
-        var result = ns.SendPipe.Reader.AvailableToRead();
+        var result = ns.SendBuffer.GetReadSpan();
         AssertThat.Equal(result, expected);
     }
 
@@ -281,7 +281,7 @@ public class AccountPacketTests
 
         ns.SendCharacterList();
 
-        var result = ns.SendPipe.Reader.AvailableToRead();
+        var result = ns.SendBuffer.GetReadSpan();
         AssertThat.Equal(result, expected);
     }
 
@@ -294,7 +294,7 @@ public class AccountPacketTests
         using var ns = PacketTestUtilities.CreateTestNetState();
         ns.SendAccountLoginRejected(reason);
 
-        var result = ns.SendPipe.Reader.AvailableToRead();
+        var result = ns.SendBuffer.GetReadSpan();
         AssertThat.Equal(result, expected);
     }
 
@@ -313,7 +313,7 @@ public class AccountPacketTests
 
         ns.SendAccountLoginAck();
 
-        var result = ns.SendPipe.Reader.AvailableToRead();
+        var result = ns.SendBuffer.GetReadSpan();
         AssertThat.Equal(result, expected);
     }
 
@@ -321,7 +321,7 @@ public class AccountPacketTests
     public void TestPlayServerAck()
     {
         var si = new ServerInfo("Test Server", 0, TimeZoneInfo.Local, IPEndPoint.Parse("127.0.0.1"));
-        var authId = 0x123456;
+        const int authId = 0x123456;
 
         var expected = new PlayServerAck(si, authId).Compile();
 
@@ -329,7 +329,7 @@ public class AccountPacketTests
 
         ns.SendPlayServerAck(si, authId);
 
-        var result = ns.SendPipe.Reader.AvailableToRead();
+        var result = ns.SendBuffer.GetReadSpan();
         AssertThat.Equal(result, expected);
     }
 }