128 effect payloadless multi arm

// Payload-less effect with a multi-arm resume sum: the yield carries nothing,
// the resume selects an arm. Emptiness is spelled by OMISSION (`! ask | ...`),
// consistent with `! pong` (void effect), `| timeout` (void arm), `| done`
// (void terminal) — never with a `()` unit token (rejected, 210_137). The
// unit appears only on the VALUE side: the proc calls `ask()` positionally,
// and the handler binds nothing (`! ask => halved 7`).
~import std/io

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

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

~request(payload: 20)
! ask => halved 7
| done r |> std/io:print.ln("{{ r:d }}")