refactor: handle kmer deduplication and layer initialization concurrently

Introduce a secondary layer iteration to open `SrcLayerData` alongside the unitig reader for concurrent metadata access. This refactors the merge routine to handle kmer deduplication and per-layer data initialization simultaneously. Also corrects a typo in `rebuild_layer.rs` where `openopen_sequential` is renamed to `open_sequential`.

src/obikpartitionner/src/rebuild_layer.rs

@@ -116,7 +116,7 @@ impl KmerPartition {
             let unitigs_path = src_layer_dir.join("unitigs.bin");
             if !unitigs_path.exists() { continue; }
 
-            let reader = UnitigFileReader::open(&unitigs_path)?;
+            let reader = UnitigFileReader::open_sequential(&unitigs_path)?;
             let src_data = SrcLayerData::open(&src_layer_dir, mode)?;
 
             for (kmer, _, _) in reader.iter_indexed_canonical_kmers() {
@@ -181,7 +181,7 @@ impl KmerPartition {
             let unitigs_path = src_layer_dir.join("unitigs.bin");
             if !unitigs_path.exists() { continue; }
 
-            let reader = UnitigFileReader::open(&unitigs_path)?;
+            let reader = UnitigFileReader::open_sequential(&unitigs_path)?;
             let src_data = SrcLayerData::open(&src_layer_dir, mode)?;
 
             for (kmer, _, _) in reader.iter_indexed_canonical_kmers() {