042 pairwise symmetric

// TEST: kernel:pairwise symmetric updates (both k and k.other writable)
//
// pairwise = true N²/2 semantics: each unique pair (i,j) visited once.
// Both elements of the pair can be updated in one visit.
// noalias on both pointers — LLVM knows ptr[i] and ptr[j] are distinct.
//
// This is the correct model for physics: force on i from j
// is equal and opposite to force on j from i.

~import std/kernel
~import std/io

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

~std/kernel:init(Body) {
    { mass: 1.0 },
    { mass: 2.0 },
    { mass: 3.0 },
}
| kernel k |> std/kernel:pairwise { k.mass += k.other.mass; k.other.mass += k.mass; }
| computed c |> std/io:print.blk {
        pairwise symmetric complete, mass[0]={{ c[0].mass:f }}
    }