Build the canonical line key from a stored booking row (matches the ${listingId}|${startAt}|${parentListingId}|${packageGroupId} identity carried by the form's hidden key field). parent_listing_id distinguishes the two rows produced when the same child is booked under two different parents; package_group_id the rows produced when the same listing is booked through two packages (or a package plus its own standalone row) in one order.