Same-listener Cowboy dispatch for native SFTP/NBD upgrades.
Mount as the catch-all Cowboy handler with %{gateway: gateway_or_mfa, fallback: {handler, handler_opts}}. Ordinary HTTP and WebSocket requests
go to the host's fallback (normally Plug.Cowboy.Handler). This module
opens no listener. Only HTTP/1.1 GET instance and storage-volume SFTP/NBD
routes can switch; all raw attempts use the shared host policy first.