BookStateFor: StorageDescriptor<
    [Key: Enum<{ Here: undefined; Parent: undefined; Sibling: number }>],
    {
        begin: number;
        count: number;
        end: number;
        message_count: bigint;
        ready_neighbours?: {
            next: Enum<{ Here: undefined; Parent: undefined; Sibling: number }>;
            prev: Enum<{ Here: undefined; Parent: undefined; Sibling: number }>;
        };
        size: bigint;
    },
    false,
    never,
>

The index of the first and last (non-empty) pages.