Function completion_popup_mode_id 

pub fn completion_popup_mode_id() -> ModeId

K.1.b (2026-05-30): canonical ModeId for the completion-popup minor-mode keymap layer. Used both by completion_popup_layer_bindings (the per-binding provenance tag at build time) and by App::sync_keymap_overlays (the push site). Centralised here so the two stay in lockstep — drift would surface as :describe-key showing the wrong mode name.