Migrate removed order coverage fields and numeric invoice coverage

v2Order.orderItemCoverage and the coverage-only fulfillment item type are removed without aliases. Queries selecting them fail validation. Read coverage on sales and fulfillment items instead.

onInvoice now returns a number, including zero when the order has no accounting documents. It no longer uses null to mean unknown. Fulfillment item quantities use the item’s product units; sales item quantities count complete bundles after pooling components. Partial bundles are not rounded up.

Removing a sales line with shipment records now preserves the line at quantity zero. Clients must not assume that a removed line disappears from the response.