Support Notify Me on metaobject-grouped products with custom availability logic

Notify Me currently follows Shopify’s native product/variant inventory only. It does not support custom themes where the PDP shows a parent/container product, the selectable options are actually separate referenced products loaded via metafields/metaobjects, and stock status on the storefront is determined by those grouped child products instead of the parent product’s Shopify inventory. In these cases, the widget should follow the availability of the referenced grouped products, not the parent product.