Function report_backend_window_id_registered 

pub fn report_backend_window_id_registered(label: String, window_id: String)

Phase B.5 (window_id_map step b) — sync API: report the frontend’s register_backend_window IPC to the launcher. Called from commands/window.rs::register_backend_window after the host’s local window_id_map insert. No-op if the launcher pipe isn’t connected.