centralize layer path construction and data loading

Replace manual directory formatting and inline matrix opening with the newly introduced `layer_dir` and `open_data` helpers from the `obilayeredmap` crate. This standardizes filesystem access, encapsulates layer naming conventions, and simplifies error handling without altering computational behavior or test suites.

src/obikindex/src/stats.rs

@@ -149,21 +149,21 @@ impl KmerIndex {
                     .unwrap_or(0);
 
                 for l in 0..n_layers {
-                    let layer_dir = index_dir.join(format!("layer_{l}"));
-                    if !layer_dir.exists() { continue; }
+                    let this_layer_dir = obilayeredmap::layer_dir(&index_dir, l);
+                    if !this_layer_dir.exists() { continue; }
 
-                    n_kmers += LayerMeta::load(&layer_dir).map(|m| m.n).unwrap_or(0);
+                    n_kmers += LayerMeta::load(&this_layer_dir).map(|m| m.n).unwrap_or(0);
 
                     let mat: Box<dyn ColumnWeights> =
-                        if layer_dir.join("counts").exists()
-                            && !layer_dir.join("presence").exists()
+                        if this_layer_dir.join("counts").exists()
+                            && !this_layer_dir.join("presence").exists()
                         {
-                            match PersistentCompactIntMatrix::open(&layer_dir) {
+                            match obilayeredmap::open_data::<PersistentCompactIntMatrix>(&index_dir, l) {
                                 Ok(m)  => Box::new(m),
                                 Err(_) => continue,
                             }
                         } else {
-                            match PersistentBitMatrix::open(&layer_dir) {
+                            match obilayeredmap::open_data::<PersistentBitMatrix>(&index_dir, l) {
                                 Ok(m)  => Box::new(m),
                                 Err(_) => continue,
                             }