Macro declare_node_app 

macro_rules! declare_node_app {
    ($app_type:ty) => { ... };
}

Generate all FFI boilerplate for a NodeApp implementation.

This macro creates:

• A OnceLock<RwLock<T>> instance for the app (sound concurrent access).
• A call gate: the shutdown entry calls NodeApp::begin_shutdown at once, refuses new calls with ERROR_SHUTTING_DOWN, and takes the write lock for NodeApp::shutdown only after in-flight calls return.
• The _node_app_entry export symbol returning a NodeAppVTable.
• FFI wrapper functions for init, shutdown, handle_request, handle_event, handle_capability, and free.
• catch_unwind guards on every FFI boundary to prevent UB from panics crossing the Rust/C ABI.
• Thread-local trace span management around handle_capability.

Invoke once at the crate root after defining the app type:

declare_node_app!(MyApp);