Render state already tracks the selected cell range for each viewport row, but C renderers could only get the full terminal selection. That required consumers to map global selection pins back into row-local spans themselves. Add row selection data to the render-state row API. Querying the new row data returns GHOSTTY_NO_VALUE for unselected rows and writes the inclusive start and end columns for selected rows. The render example now demonstrates setting a selection and reading the row-local range while iterating rows.