Snap PDF float colours to palette when classifying fills

PDF fill colours are floats, so int(v*255) and int(round(v*255)) give hex values that differ by one, such as #ff642a vs #ff652b. An exact-match lookup then silently misses most fills; in this case only the yellow tile matched. Snap each extracted colour to the nearest colour in a known brand palette with a small tolerance before grouping or selecting by colour.