018 optional effect noop discard banned

// RULING 1: a no-op `_` body on an OPTIONAL effect branch is ILLEGAL.
// `! ?tick` is optional; `! tick _ |> _` subscribes only to do nothing —
// pure noise (just omit the handler), AND a hazard: if `tick` is later
// promoted to REQUIRED, a no-op handler would silently swallow the event
// instead of surfacing the must-handle error.
//
// STILL LEGAL (pinned elsewhere): a no-op `_` on a REQUIRED effect branch
// (explicit handle-and-ignore carries meaning), and `! tick _ |> <action>`
// (discard the payload but actually act).
~import std/io

~pub tor poll { x: i64 }
! ?tick i64
| done

~proc poll|zig { return .{ .done = .{} }; }

~poll(x: 1)
! tick _ |> _
| done |> _