Derive the Binary instance a client's pattern (binder) type needs, alongside the hand-written instances of Control.Monad.Free.Foil.Binary.

A pattern type is a GADT over two scope indices, so its instance cannot come from GHC.Generics. What the deriver writes is the shape one would write by hand: one tag byte per constructor in declaration order, then the fields in order. Decoding happens at the diagonal, with every scope index of a constructor instantiated to the same variable, which any chain of binder indices admits. A single coercion then moves the result to the requested indices. That coercion mints scope evidence, so a derived instance is part of the same trust boundary as the library's own. See the module documentation of Control.Monad.Free.Foil.Binary.