Selection-State Class for an Infinite-Scroll Checkbox List with Select All

Implement a state management class for an infinite-scroll list where each row has a checkbox, and a header checkbox controls the selection of all rows at once. The class is responsible for fetching pages as the user scrolls, maintaining per-row checked state, and coordinating the header checkbox's three visual states: fully checked, fully unchecked, and partially checked. The UI layer will subscribe to state snapshots and call methods like loadMore(); actual scroll detection…