SIMD sum forms

# `simd.sum` is worked out in full and checked where it lands, as any plain number; `simd.sum.lane`
# checks it at the lane's type, and `simd.sum.wrap` wraps it there as the chip's own sum does, never a
# check (owner, 2026-10-07: "There should be a way to do the other option too", then "yea").
MAIN {
    let 'v': simd[int32, 4] = simd.fill[value 1200000000];
    let 's': int64 = simd.sum['v'];
    let 'w': int32 = simd.sum.wrap['v'];
    let 'u': simd[int32, 4] = simd.fill[value -500000000];
    let 'l': int32 = simd.sum.lane['u'];
    std::print.useful['s', " ", 'w', " ", 'l', newln];
}