ConstExists so a non-component owner — e.g. the cell IEditable publishing its active session — can drive the bus from anywhere, not only where a hook happens to be mounted. Reads (and the React subscription) still go through useEditMode; this is write-only. Before any component has called useEditMode there is no bus, and the write is dropped.
Publish (or clear) the active EditMode on the app-wide bus without a hook — the same write useEditMode's
setModeperforms (fires the superseded mode's EditMode.onModeChange), callable from plain (non-React) code.