feat: wire shared nic into instance setup

Route normal, DHCP, and mobile nic creation through one helper that
selects the shared backend when dev_name is configured.

Keep dedicated nic creation as the default for configs without a
dev_name, and keep no_tun handling in the existing callers.

Store a process-wide shared virtual nic registry behind each instance
so networks with the same dev_name attach to the same shared device.

easytier/src/launcher.rs

@@ -121,6 +121,7 @@ impl EasyTierLauncher {
         let peer_mgr = instance.get_peer_manager();
         let nic_ctx = instance.get_nic_ctx();
         let peer_packet_receiver = instance.get_peer_packet_receiver();
+        let shared_virtual_nic_registry = instance.get_shared_virtual_nic_registry();
         let mut tun_fd_receiver = data.tun_fd.1.lock().unwrap().take().unwrap();
 
         tasks.spawn(async move {
@@ -133,6 +134,7 @@ impl EasyTierLauncher {
                     global_ctx.clone(),
                     peer_mgr.clone(),
                     peer_packet_receiver.clone(),
+                    shared_virtual_nic_registry.clone(),
                     tun_fd,
                 )
                 .await;