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/building.md

@@ -0,0 +1,54 @@
+# Building
+
+## Go
+
+Requires Go 1.24 or later.
+
+```bash
+go build -o softether-go ./cmd/softether-go/
+```
+
+### Static binary
+
+For deployment in minimal containers (Alpine, scratch) or systems without glibc:
+
+```bash
+CGO_ENABLED=0 go build -o softether-go ./cmd/softether-go/
+```
+
+### Cross-compilation
+
+```bash
+CGO_ENABLED=0 GOOS=linux GOARCH=amd64 go build -o softether-go ./cmd/softether-go/
+CGO_ENABLED=0 GOOS=linux GOARCH=arm64 go build -o softether-go ./cmd/softether-go/
+```
+
+Only Linux targets are supported (the TAP device interface uses Linux-specific ioctls).
+
+## Nix
+
+Build with the flake:
+
+```bash
+nix build
+```
+
+The result is in `./result/bin/softether-go`.
+
+Enter a development shell with Go tooling:
+
+```bash
+nix develop
+```
+
+Build within the dev shell:
+
+```bash
+nix develop -c bash -c 'go build -o softether-go ./cmd/softether-go/'
+```
+
+## Dependencies
+
+The only Go dependency is `golang.org/x/sys` for Linux syscall constants (TUN/TAP ioctls). Dependencies are vendored in `vendor/`.
+
+At runtime, the only external dependency is `ip` (iproute2) for route and address management.