test(ipv6): reuse the TUN registry across client reconnects (#2765)

The public IPv6 reconnect fixture kept its process runtime but created
a fresh shared-TUN registry for each client. A replacement could open
the same named device while the old registry's asynchronous cleanup
still owned it, failing with EBUSY in CI.

Keep one registry in PublicIpv6Lab and use it for initial and replacement
instances, matching NativeInstanceFactory. Existing registration IDs
and the shared NIC mutex coordinate cleanup and replacement. Preserve
the lease, route, and ping assertions without changing their timeouts.

Validate with 20 reconnect stress runs, 4 public IPv6 cases, 7 shared-TUN
integration cases, formatting, and strict workspace clippy. The baseline
also passed 30 local runs, so this is not a deterministic local red/green
reproduction. Runtime stop semantics are unchanged.

easytier/src/tests/three_node.rs

@@ -457,6 +457,8 @@ fn connect_ns_to_bridge(ns: &str, guest_if: &str, host_if: &str, bridge: &str) {
 
 struct PublicIpv6Lab {
     network: TestNetwork,
+    #[cfg(feature = "tun")]
+    shared_virtual_nic_registry: crate::instance::shared_virtual_nic::ArcSharedVirtualNicRegistry,
 }
 
 #[derive(Clone, Copy)]
@@ -643,7 +645,30 @@ impl PublicIpv6Lab {
             "net.ipv6.conf.all.forwarding=1",
         );
 
-        Self { network }
+        Self {
+            network,
+            #[cfg(feature = "tun")]
+            shared_virtual_nic_registry: Instance::new_shared_virtual_nic_registry(),
+        }
+    }
+
+    fn new_instance(
+        &self,
+        config: TomlConfigLoader,
+        process_runtime: Arc<CoreProcessRuntime>,
+    ) -> Instance {
+        // Reconnect through the same registry, as NativeInstanceFactory does,
+        // so old member cleanup and new registration share the TUN owner.
+        #[cfg(feature = "tun")]
+        {
+            Instance::new_with_process_runtime_and_shared_virtual_nic_registry(
+                config,
+                process_runtime,
+                self.shared_virtual_nic_registry.clone(),
+            )
+        }
+        #[cfg(not(feature = "tun"))]
+        Instance::new_with_process_runtime(config, process_runtime)
     }
 }
 
@@ -702,8 +727,8 @@ async fn init_public_ipv6_two_node_with_topology(
     );
     client_cfg.set_ipv6_public_addr_auto(true);
 
-    let mut provider = Instance::new_with_process_runtime(provider_cfg, process_runtime.clone());
-    let mut client = Instance::new_with_process_runtime(client_cfg, process_runtime.clone());
+    let mut provider = lab.new_instance(provider_cfg, process_runtime.clone());
+    let mut client = lab.new_instance(client_cfg, process_runtime.clone());
 
     provider.run().await.unwrap();
     client.run().await.unwrap();
@@ -985,7 +1010,7 @@ pub async fn public_ipv6_auto_addr_reconnect_reuses_same_address() {
         client_id,
     );
     client_cfg.set_ipv6_public_addr_auto(true);
-    let mut client = Instance::new_with_process_runtime(client_cfg, process_runtime.clone());
+    let mut client = lab.new_instance(client_cfg, process_runtime.clone());
     client.run().await.unwrap();
     provider.add_connector_url("tcp://10.1.1.2:11010".parse().unwrap());