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

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