ClassifyItems() layers ApplyChainGrouping on top of partitioning, which
writes ShowAsChainMember/ChainStep/ChainAfterLabel on every row.
ClearCompletedAsync only needed the completed-section count/rows, so it
got that mutation as an unwanted side effect of counting.
Split the pure overdue/open/completed partition into PartitionItems();
ClassifyItems() now composes it with the chain-grouping mutation for
Regroup's actual render path. ClearCompletedAsync uses PartitionItems
directly. Regroup's behavior is unchanged.
