Sidecar chains {content,name,ext} specs over a SIDECAR-typed link; the save node
mirrors SaveImageKJ (folder_paths.get_save_image_path) and writes each sidecar as
base+name+ext next to the image. build_plan validates the whole chain before any
I/O so duplicates/bad extensions write nothing.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
