Rename the KmerPartition type to KmerPartitions across obikindex, obikpartitionner, and obikphylo/siblings to reflect an updated data model. Update the Mat enum in siblings/cache.rs to add a SparsePresence variant and simplify opening logic by delegating sparse versus dense detection to PersistentBitMatrix. Apply consistent code formatting, import reordering, and multi-line refactoring throughout the affected modules.