Subitems: records inside records — Nesting several levels

Subitems nest. If serial numbers are subitems of line items, and line items are subitems of orders, then each line item row in the order gets an expander (▸). Open it and that line item's serial numbers appear, with their own Add button. Levels load only when you expand them, so a long order stays fast. Nesting is capped at four levels deep — past that, open the row itself (↗) to keep going.