feat: Windows stack (daemon, installer, GUI)

Components:
- ostp-daemon: Windows Service with Named Pipe IPC
- ostp-installer: Setup wizard with admin privileges
- ostp-gui: Tauri dark theme UI (450x600)

Features:
- Background service management (OspabGuard)
- IPC commands: CONNECT/DISCONNECT/STATUS
- Firewall rules auto-configuration
- Wintun driver placeholder (download from wintun.net)
- Real-time stats display (upload/download/ping)

Note: Requires wintun.dll download for full functionality

ostp-installer/assets/README-wintun.md

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+# Wintun DLL Placeholder
+
+**NOTE**: This is a placeholder file. You need to download the actual `wintun.dll` from:
+https://www.wintun.net/
+
+**Instructions:**
+1. Download Wintun from https://www.wintun.net/
+2. Extract the archive
+3. Copy `wintun/bin/amd64/wintun.dll` to this directory
+4. The installer will embed this DLL at compile time using `include_bytes!`
+
+**License**: Wintun is dual-licensed under GPLv2 and a commercial license.

ostp-installer/assets/wintun.dll

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+// Placeholder wintun.dll
+// Download actual file from https://www.wintun.net/
+// This file is here to allow compilation without the actual DLL