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. Requires the bus singleton
to be mounted (a SingletonHooksContainer in the tree), exactly like the hook.
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setModeperforms (fires the superseded mode's EditMode.onModeChange), callable from plain (non-React) code.