Variants missing item_group_id

Color/size variants must share an item_group_id so Google groups them as one product. Without it, Google treats every variant as a standalone product, hurting visibility.

Feed exports all variants but doesn't include the parent grouping ID. Each variant gets its own slot in Shopping rather than rolling up.

1. Add item_group_id to every variant — same value for all variants of one product (often the parent SKU).
2. Each variant should still have its own unique id.
3. Apparel must use item_group_id; not optional in apparel feeds.

• Reusing id as item_group_id (they should differ)
• Different item_group_id per color when sizes should also be grouped