Variants missing item_group_id

Without item_group_id, Meta treats each variant as a separate product instead of rolling them up. Conversion suffers because shoppers don't see the variant picker on Shop pages.

Variant exports don't tag the parent grouping ID; Meta sees N independent products instead of one product with N variants.

1. Set item_group_id on every variant — same value across siblings (often the parent SKU).
2. Each variant still keeps its own unique id.
3. Apparel especially needs this — without it, color/size pickers won't render.

• Using id as item_group_id (they should differ)