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/obikphylo/src/siblings/family_scan.rs

@@ -54,6 +54,7 @@ use std::sync::atomic::{AtomicU8, Ordering};
 use rayon::prelude::*;
 
 use obikseq::CanonicalKmer;
+use obilayeredmap::layer_dir;
 use obilayeredmap::meta::PartitionMeta;
 use obipipeline::{ThrottleGuard, throttle};
 
@@ -111,15 +112,15 @@ pub(crate) fn sibling_layer_dirs(index: &KmerIndex) -> OKIResult<Vec<PathBuf>> {
         }
         let meta = PartitionMeta::load(&index_dir).map_err(olm_to_ok)?;
         for l in 0..meta.n_layers {
-            let layer_dir = index_dir.join(format!("layer_{l}"));
-            let annex_path = layer_dir.join(ANNEX_FILE_NAME);
+            let this_layer_dir = layer_dir(&index_dir, l);
+            let annex_path = this_layer_dir.join(ANNEX_FILE_NAME);
             if !annex_path.exists() {
                 return Err(OKIError::InvalidInput(format!(
                     "no sibling annex at {} — run build_sibling_annex first",
                     annex_path.display()
                 )));
             }
-            layer_dirs.push(layer_dir);
+            layer_dirs.push(this_layer_dir);
         }
     }
     Ok(layer_dirs)