Answer
In a table's edit mode, Tab moves focus to the next cell in cell-edit mode (which disables plain arrow-key cell navigation, wrapping to the next row once you tab past the last column); to move between rows more directly, press Enter first to exit cell-edit mode, which re-enables the arrow keys for navigating between cells/rows, then Enter again (or start typing) to re-enter edit mode on the newly focused cell. Shift+Tab, Esc, and Alt+S (save) round out the same table's supported keyboard shortcuts.