The editable (UI model) — the same instance the SheetCellEditorLayer renders.
OptionalonNotified when a start-edit attempt is rejected (startEdit returned false — a non-editable
cell). The host that mounts one layer per pane (SheetElement) passes a SHARED callback
(useEditRejectedNotice) so all panes form one notice scope; it receives the rejected cell.
this layer's own useEditRejectedNotice
The sheet — stamps the double-clicked cell's scope (sheet name) onto the coords it starts on.
Props for SheetCellEditorStartLayer.