Moves keyboard focus to the next annotation in tab order (Tab): within a page in its Tabs order, then on to the next loaded
page. The accessibility seam: a screen-reader shell follows Focused and reads GetAccessibleName(AnnotationTarget).
public bool FocusNext()
ISO 32000-2 §12.5.1 and Table 31 (Tabs: R row order, C column order, A annotation order; S structure order
falls back to annotation order).