126 reject unknown resume arm

// Multi-arm resume: the `=>` construct must name one of the DECLARED arms.
// `tripled` is not in `ask`'s resume sum { halved, timeout }; without this
// wall the unknown arm sails through to a raw Zig error in the emitted union
// construction. The koru-level diagnostic lists the legal arms.
~import std/io

~pub tor request { payload: i32 }
! ask i32
    | halved i32
    | timeout
| done i32

~proc request|zig {
    const r = ask(payload);
    return switch (r) {
        .halved => |v| .{ .done = v },
        .timeout => .{ .done = -1 },
    };
}

~request(payload: 20)
! ask n => tripled n
| done r |> std/io:print.ln("{{ r:d }}")