Function shutdown_data_plane_runtimes 

pub fn shutdown_data_plane_runtimes(timeout: Duration)

Shut down every data-plane runtime built by build_data_plane_runtime, waiting up to timeout for each. Idempotent: a second call finds an empty registry and does nothing. Pure Rust (no Python); the pyo3 layer must call this at Python teardown, before Py_Finalize, so data-plane workers stop before the interpreter is finalized.