Linux backend for Net Lattice: implements net-lattice-platform's provider
traits via Netlink.
Only ever compiled for target_os = "linux" — its dependencies
(rtnetlink, Linux-only) are gated the same way in Cargo.toml. See
ARCHITECTURE.md for how this crate binds net-lattice-platform's generic
RouteProvider::Route associated type to the concrete
net_lattice_model::route::Route.