ConstExported so the argument FORM can trail the same cluster after each of its labels: an argument carries the same three facts wherever it is met, and the icons are how they are said without spending a line of prose on each.
TODO (scheme still to finalize): Repeat for a rest parameter, DataArray (the […] brackets) for
an OPTIONAL slot, and DataObject as the generic value/type marker. The scalar-type → icon mapping
(number / text / logical / lambda …) is the part we still want to design.
The per-argument metadata icons — repeatable / optional / value type, each with a native tooltip.