fold subflow union outputs

// PIN — a `#label` fold whose implementing subflow declares UNION outputs:
// the back-edge branch loops inside `while`, the remaining arms construct the
// impl's output union after it, and the after-loop switch names the looped
// branch `unreachable` (Zig does not narrow union(enum) through the guard).
const std = @import("std");
~import std/io

~tor round { text: string, rounds: i32 }
| again { text: string, rounds: i32 }
| stop string
| broke
~proc round|zig {
    if (rounds < 0) return .broke;
    if (rounds <= 1) return .{ .stop = text };
    return .{ .again = .{ .text = text, .rounds = rounds - 1 } };
}

~tor polish { text: string, rounds: i32 }
| polished string
| broke

~polish = #loop round(text, rounds)
| again a |> @loop(a.text, a.rounds)
| stop out => polished out
| broke => broke

~polish(text: "hi", rounds: 3)
| polished out |> std/io:print.ln("{{ out:s }}")
| broke |> std/io:print.ln("broke")