display_rgb flipped every near-white colour to black on paper and every
near-black colour to white on the dark canvas, so ACI 51/255 and
true-colour white printed black and dark index colours showed white.
Display-list primitives now carry an aci7 flag set when the colour
resolves to 7 (directly, by layer or by block); only those flip.
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
