SplitProduct: show variants as separate products

Show every color, size, material or style as its own product card in collections, search results and homepage sections.

• Split by any option. Color, size, material or one card per variant, with no duplicate products and no theme code changes.
• Merchandise each collection. Reorder variant cards with drag and drop, hide them, and customize titles and hover images.
• Smart filtering. Hide out-of-stock variants, show discounted variants only, mix the order, or build collections of specific variants.