Replace manual layer path construction with obilayeredmap::layer_dir

Standardizes layer directory path resolution across obikindex, obikpartitionner, and obikphylo by replacing inline string formatting and join calls with the centralized obilayeredmap::layer_dir utility. This refactoring removes redundant path construction logic while preserving existing iteration bounds, control flow, and public APIs.

src/obikindex/examples/compare_sparse.rs

@@ -5,6 +5,7 @@
 
 use obicompactvec::{PersistentBitMatrix, PersistentSparseBitMatrix};
 use obikindex::KmerIndex;
+use obilayeredmap::layer_dir;
 
 fn main() -> anyhow::Result<()> {
     let sparse_root = std::env::args().nth(1).expect("usage: compare_sparse <sparse_root> <dense_root>");
@@ -33,8 +34,10 @@ fn main() -> anyhow::Result<()> {
         }
 
         for layer in 0..n_layers {
-            let layer_dir_sparse = part_dir_sparse.join("index").join(format!("layer_{layer}"));
-            let layer_dir_dense = part_dir_dense.join("index").join(format!("layer_{layer}"));
+            let index_dir_sparse = part_dir_sparse.join("index");
+            let index_dir_dense = part_dir_dense.join("index");
+            let layer_dir_sparse = layer_dir(&index_dir_sparse, layer);
+            let layer_dir_dense = layer_dir(&index_dir_dense, layer);
 
             if !layer_dir_sparse.exists() || !layer_dir_dense.exists() {
                 continue;