Variant reference metafields use handle.title format

Moving variant-linked data between stores is now simpler and more readable.

🔧 Improvements

• Variant reference exports – now use [product handle].[variant title] for clearer spreadsheets and easier cross-store moves (Metafields reference).
• Variant reference imports – accept the same handle.title format; escape commas in variant titles with a backslash (e.g., Red \, Large) (Metafields reference).
• Lists supported – works for list.variant_reference columns as well.