Table cell selection

We have row selection, but not cell selection. We should be able to move left/right to highlight a specific cell and make that information available to parent components in the same way as the row.

Start by reading the table component's existing row-selection behavior and its parent-component API. Trace the left/right navigation entry point, then determine how cell-selection state should be exposed alongside row selection; done means a cell can be highlighted horizontally and its information is available to parent components in the same way as a row.

Written by the indexing model from the issue text.