refactor: streamline merge pipeline and MPHF indexing

Replace mphf.find() with direct mphf.index() calls to eliminate absence checks and fallback vectors. Introduce a lightweight MphfOnly wrapper for faster index loading, and standardize k-mer iteration across merge and rebuild layers. Update IndexMeta configuration and n_new calculation to leverage MPHF cardinality, streamlining the overall merge pipeline.

src/obikpartitionner/src/rebuild_layer.rs

@@ -117,7 +117,7 @@ impl KmerPartition {
             if !unitigs_path.exists() { continue; }
 
             let reader = UnitigFileReader::open_sequential(&unitigs_path)?;
-            let src_data = SrcLayerData::open(&src_layer_dir, mode, &src_meta.mode)?;
+            let src_data = SrcLayerData::open(&src_layer_dir, mode)?;
 
             for (kmer, _, _) in reader.iter_indexed_canonical_kmers() {
                 let row = src_data.lookup(kmer, n_genomes);
@@ -182,7 +182,7 @@ impl KmerPartition {
             if !unitigs_path.exists() { continue; }
 
             let reader = UnitigFileReader::open_sequential(&unitigs_path)?;
-            let src_data = SrcLayerData::open(&src_layer_dir, mode, &src_meta.mode)?;
+            let src_data = SrcLayerData::open(&src_layer_dir, mode)?;
 
             for (kmer, _, _) in reader.iter_indexed_canonical_kmers() {
                 let row = src_data.lookup(kmer, n_genomes);