Introduces `index` and `index_batch` methods for direct MPHF slot mapping without membership validation, alongside four public iterator methods for deterministic traversal of canonical kmers. These are backed by dedicated `KmerIter` and `KmerBatchIter` structs that wrap the underlying unitig file reader. Updates `LayerEvidence::Approx` to eagerly open the unitig reader during initialization, enforcing a clear separation between raw mapping and verified lookup workflows.
