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/obikseq/Cargo.toml

@@ -3,6 +3,11 @@ name = "obikseq"
 version = "0.1.0"
 edition = "2024"
 
+[features]
+# Replaces the OnceLock-based params with thread-local storage so that
+# tests in dependent crates can call set_k / set_m freely without conflicts.
+test-utils = []
+
 [dependencies]
 bitvec = "1"
 serde = { version = "1.0", features = ["derive"] }