Integrate rayon to enable parallel processing of k-mer partitions and degree computation. Replace Cell with AtomicU8 to ensure thread-safe node state management, and add a merge method for combining disjoint graphs. Additionally, introduce progress tracking utilities and a test-utils feature flag for development dependencies.
Centralize k-mer and minimizer configuration using a thread-safe global module, and replace manual bit-packing with a memory-efficient `PackedSeq` type. Refactor core sequence and k-mer types to use compile-time length enforcement and centralized hashing. Introduce a new De Bruijn graph implementation with compact node encoding and traversal iterators. Update I/O, partitioning, and builder modules to align with the new architecture, and add the `xxhash-rust` dependency.