Run demo.py --with-gui gradio (Dockerfile CMD); add gradio + scipy to
requirements; refresh README. Sync the optgs source + demo.py to the current
upstream-merged tree (the gradio GUI: live decoder-render stream + interactive
Model3D, with the Z-up->Y-up splat reorientation and the thread-safe
optimize_iter). Prebuilt wheels and submodules are unchanged.
