Splits sampled frames into GOP groups. Each group produces exactly one GOP canvas: the group's first frame stays whole as the I-frame, and each later frame gets its own P section below it. So GOP=4 means each group is I P P P. For fixed GOP=N, the number of packed canvases is ceil(sampled_frames / N). Example: 32 sampled frames with GOP=4 gives 8 canvases; 32 sampled frames with GOP=8 gives 4. Codec-stream mode adaptively groups by saliency energy, targeting roughly 8-64 sampled frames per group.