116 effect resume arrow producer

// The canonical resume-value producer: an effect declared `-> T` (single
// payload, no branches) has its resume value PRODUCED with `->`, the
// function-like produce glyph. NOT `|>` (which only chains a step) and NOT
// `=>` (which constructs a branch — illegal here, the effect has no branches).
//
// This pins R2 for resume values: the glyph is fixed by the effect's
// declaration. `! ask []const u8 -> i32` is a singular-payload effect, so the
// arm produces with `-> 42`. Mirror of the event-side bare return (020_021).
~import std/io

~pub event query { q: []const u8 }
! ask []const u8 -> i32
| done i32

~proc query|zig {
    const a = ask(q);
    return .{ .done = a };
}

~query(q: "x")
! ask _ -> 42
| done r |> std/io:print.ln("{{ r:d }}")