030 pairwise outer range

// TEST: kernel:pairwise with outer range expression
//
// pairwise(0..N) { body } |> per_step(...)
//
// Verifies:
// 1. The outer loop is generated by pairwise, not a separate ~for
// 2. The per-step continuation runs INSIDE the outer loop (N times, not 1)
// 3. Ergonomics: one less level of nesting vs for | each _ |> pairwise

~import "$std/kernel"
~import "$std/io"

~std.kernel:shape(Body) {
    mass: f64,
}

~std.kernel:init(Body) {
    { mass: 1.0 },
    { mass: 2.0 },
}
| kernel k |>
    std.kernel:pairwise(0..3) {
        k.mass += k.other.mass
    }
    |> std.io:print.ln("step")

~std.io:print.ln("done")