WinTun: ring buffer transport

[benaadams]

Problem

Replace the FileStream-based implementation with the native ring buffer API for true zero-copy I/O.

Sub-tasks

• WinTunNativeTransport implementing IPacketTransmitter and IPacketReceiver
• Receive loop: WintunReceivePacket -> process -> WintunReleaseReceivePacket
• Send path: WintunAllocateSendPacket -> write into buffer -> WintunSendPacket (true zero-copy)
• Adapter lifecycle: create on init, destroy on dispose
• Thread safety: document single-producer/single-consumer constraints from WinTun API
• Keep old TAP implementation available behind a flag or as a separate class (don't delete yet)

Dependencies

Depends on: #115
Unlocks: #117 (update sample + benchmark), #118 (WinTun tests), #132 (Kestrel Windows backend), #137 (API docs)

Phase

Phase 2 (Platform Backends & Samples)