#154Brightness-Sorted Wipe

A cratered, dramatically-lit planet (sourceA) — hard specular dayside, deep terminator, black-space background, i.e. strong luminance structure — dissolves into a vivid magenta/cyan/gold nebula (sourceB) in luminance order. mask = smoothstep(thr-e, thr+e, luminance(A)) with the threshold sweeping so B emerges from the brightest highlights first and floods down into the shadows, then eases back. A glowing front traces the moving iso-luminance contour where A is handing over to B. Loops autonomously.