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/Server.Tests.csproj

@@ -16,6 +16,13 @@
         <DataFiles Include="$(SolutionDir)\Distribution\Data\**" />
         <ProjectReference Include="..\UOContent\UOContent.csproj" />
     </ItemGroup>
+    <!-- Copy native ioring.dll for tests -->
+    <ItemGroup>
+        <Content Include="C:\Repositories\IORingGroup\IORingGroup\runtimes\win-x64\native\ioring.dll" Condition="Exists('C:\Repositories\IORingGroup\IORingGroup\runtimes\win-x64\native\ioring.dll')">
+            <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
+            <Link>ioring.dll</Link>
+        </Content>
+    </ItemGroup>
     <Target Name="CopyData" AfterTargets="AfterBuild">
         <Copy SourceFiles="@(DataFiles)" DestinationFolder="$(OutDir)\Data\%(RecursiveDir)" />
     </Target>