039 branch payload requires binding

// PINS: a branch that carries a payload must be bound or explicitly discarded.
//
//   | result r |>    binds the payload to 'r'
//   | result _ |>    explicitly discards it
//   | result |>      names the branch and drops the payload on the floor
//
// The third form is the one refused, and the refusal must name the branch.
//
// Two branches, not one: a lone payload-carrying branch is refused at the
// DECLARATION by PARSE003 ("declare the single output as a bare return
// instead"), which masks this case entirely at one branch.
const std = @import("std");
~import std/io

~tor compute { x: i32 }
| result i32
| skipped

~compute => result x * 2

~compute(x: 42)
| result |> std/io:print.ln("done")
| skipped |> _