116 reduce self under step

// TEST: std/kernel — `reduce` composes with `self` UNDER `step` in one init.
//
// Each step mutates every mass (self), then reduce aggregates the mutated state
// through DECLARED accumulator slots (`<name>[<type>]`). This is the realistic
// shape a simulation needs: evolve the data AND answer a question about it in
// the same pass. Expected: self mass+=1.0 over 3 steps of masses [1,2] yields
// per-step sums [5,7,9] → total=21 count=6.

import std/kernel
import std/io

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

std/kernel:init(Pt) {
    { mass: 1.0 },
    { mass: 2.0 },
}
| kernel k |> std/kernel:step(0..3)
    | step |> std/kernel:self { k.mass += 1.0 }
        |> std/kernel:reduce {
            total_mass[f64]
            count[i32]
            total_mass += k.mass
            count += 1
        }
| reduce r |> std/io:print.blk {
        total={{ r.total_mass:f }} count={{ r.count:d }}
    }