Subitems: records inside records — The grid on the parent record

Open any parent record and the subitems appear as a grid below its fields, with an Add… button. Rows are editable in place — click a cell, type, tab onward. Computed columns ride along read-only, so a per-line lookup or rollup is visible while you work. The link back to the parent isn't shown as a column: it's always this record, so showing it would just be noise. A relation column inside the grid (a line item's part number, say) gets a picker that searches the target table — and if what you need isn't there yet, type the name and choose Create to make it and link it without leaving the row.