Change what a single variant sends to one channel

Until now an override changed a product on one connection, and every variant went out with the same numbers everywhere. Open a variant, choose Overrides from its menu, and pick the connection: you can now set that channel's own Identifiers, Fallback Stock, Product and Shipping dimensions, Country of Origin and HS Code. A product with no variants shows these fields in the product's own Overrides. Each connection offers only the fields its channel accepts.