Difference visualization

Either
- by showing the two layers (layer groups) next to each other, with control points connected by lines (as suggested by Volker), similar to the elephant example in https://ch.mathworks.com/discovery/image-registration.html, or
- by showing the difference between two images `diff_img = np.abs(img1 - img2)` (this only works for images from the same modality/contrast range, though)

No file, test, or entry point is named. Review the linked image-registration example and the current visualization behavior, then resolve whether the implementation should show aligned layers with connected control points or an absolute image difference. Done means one of these comparison views is available and works within the stated modality limitation.

Written by the indexing model from the issue text.