refactor: centralize k-mer config and introduce packed sequences

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.

src/obidebruinj/Cargo.toml

@@ -8,3 +8,7 @@ obikseq = { path = "../obikseq" }
 obifastwrite = { path = "../obifastwrite" }
 ahash = "0.8"
 hashbrown = "0.14"
+xxhash-rust = { version = "0.8.15", features = ["xxh3", "const_xxh3"] }
+
+[dev-dependencies]
+obikseq = { path = "../obikseq", features = ["test-utils"] }