Add the workspace crate that will host a pure Rust Headscale/Tailscale- compatible VPN client. This first commit lands the crate skeleton plus the leaf modules that the rest of the stack builds on: - error: thiserror Error enum + Result alias - config: VpnConfig - keys: Curve25519 node/disco/wg key types with on-disk persistence - proto/types: PascalCase serde wire types matching Tailscale's JSON