The Elmish Model state used to manage input boxes that can be used to define parameter expressions. Part of Model.PopupDialogData.DialogState. Keyed by ParamSlot rather than CompSlotName, so that the key names WHICH component's box this is. Slot names are shared by construction - every component with a width has `Buswidth`, every instance of one sheet has `CustomCompParam "w"` - so keying on the name alone meant one component's box read another's entry, and selecting a second component showed the first's error on it in red. A popup has no component and uses an empty CompId; its entries are cleared wholesale when the popup closes.