Function computeListWindow

Computes the render plan for a windowed list.

Invariants (relied on by the Go-To dialog's accessibility contract):

• Option rows plus spacer rows always sum to exactly totalCount, so the scrollbar geometry matches a fully-rendered list.
• A valid activeIndex is always present as an option row, wherever the window is — aria-activedescendant must reference a mounted element.
• Option indices are strictly increasing with no duplicates.