017 subflow produces named branch

// Pins: a flow that implements a tor with NAMED BRANCHES may produce one of
// those branches from a terminal arm — `-> finished d` where the implemented
// tor declares `| finished string`.
//
// The bare-return twin (`-> expr` on a `-> T` tor) was always emitted as
// `return expr;`. A branchy tor had no counterpart, so the produce fell through
// to a discard, `_ = finished d;`, which is not valid Zig — meaning a flow
// could only ever implement a bare-return tor. Both arms are exercised here:
// a payload branch and an empty one.

~import std/io

~tor source { n: i32 }
| done string
| empty

~proc source|zig {
    if (n == 0) return .empty;
    return .{ .done = "ok" };
}

~tor drive { n: i32 }
| finished string
| blank

~drive =
    source(n)
    | done d -> finished d
    | empty -> blank

~drive(n: 1)
| finished f |> std/io:print.ln(f)
| blank |> std/io:print.ln("blank")

~drive(n: 0)
| finished f |> std/io:print.ln(f)
| blank |> std/io:print.ln("blank")