feat: support shared nic mobile fd

Create the shared virtual nic dispatcher from the mobile tun fd.

Later shared members attach through their ring tunnel and do not
consume another fd. Dedicated mobile nic creation stays unchanged.

Fix mobile no-magic-dns cfg so the mobile tun path checks without
enabling magic-dns.

easytier/src/launcher.rs

@@ -128,7 +128,7 @@ impl EasyTierLauncher {
                 let Some(tun_fd) = tun_fd_receiver.recv().await.flatten() else {
                     return;
                 };
-                let res = Instance::setup_nic_ctx_for_mobile(
+                let _ = Instance::setup_nic_ctx_for_mobile(
                     nic_ctx.clone(),
                     global_ctx.clone(),
                     peer_mgr.clone(),