Pages reduced by k > 1 whose source starts on a multiple of k now use the
band/tile reduced render (one-colour tile skipping, parallel tiles) on a
factor-aligned cell grid instead of compositing every pixel at full size and
averaging. Unaligned sources keep the averaging path, which tests use as the
reference (U8/U16/F32, k = 2, 4, 16, max difference < 1e-4). The reduced
render now checks the page's interrupt between bands.