perf: eliminate tokio::sync::Mutex across async boundaries in stub and nvim-core

stub/tests/e2e.rs

@@ -61,7 +61,7 @@ async fn test_full_system_e2e_performance() {
     assert!(stub_exe.exists(), "Stub not found at {:?}", stub_exe);
 
     // 1. Start Server
-    let mut server = ChildGuard(Command::new(&server_exe)
+    let _server = ChildGuard(Command::new(&server_exe)
         .arg("--daemon")
         .env("MCP_PORT", test_port)
         .env("RUST_LOG", "debug")