initial commit: standalone SoftEther VPN client in Go

Built-in DHCP (raw Ethernet frames through tunnel), automatic reconnection,
host route management, classless static routes (option 121/249), DNS config.
Single static binary, Linux only.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/main.md

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+# softether-go documentation
+
+Standalone SoftEther VPN client written in Go. Connects to SoftEther VPN servers using the native protocol over TLS, with built-in DHCP, automatic reconnection, and route management.
+
+## Features
+
+- Native SoftEther protocol (TLS + HTTP handshake + TCP block framing)
+- Built-in DHCP client (raw Ethernet frame construction through the VPN tunnel)
+- Automatic reconnection with fresh DHCP on each reconnect
+- Host route to VPN server via existing default gateway (prevents routing loops)
+- Classless static routes (DHCP option 121/249, RFC 3442)
+- DNS configuration from DHCP lease (backup/restore of `/etc/resolv.conf`)
+- Hashed password (SHA-0) and plaintext password (RADIUS/external) authentication
+- Single static binary, Linux only
+
+## Contents
+
+- [Usage & CLI reference](usage.md) — flags, examples, Docker usage
+- [Architecture](architecture.md) — connection flow, DHCP, reconnection, routing
+- [Building](building.md) — Go, Nix, static builds
+- [Project structure](structure.md) — source layout and package descriptions
+
+## Requirements
+
+- Linux (uses `/dev/net/tun` for TAP devices)
+- `CAP_NET_ADMIN` or root (TAP device creation, route management)
+- `ip` command (iproute2) on `$PATH`