Field mode (modality)

How a stored slot relates to the mode of the value that contains it. Distinct from Ship's function channels (exn, eff).

One @ on the slot — a product field, a variant payload, or a type parameter when every stored occurrence shares the cut. Not OxCaml's @@. The executable chapter is Field modes. The axes are in ../16-modes.md. Open residue: questions/102-modality-syntax.md.