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/build.rs

@@ -7,7 +7,7 @@ use rayon::prelude::*;
 use obikpartitionner::KmerPartition;
 use obipipeline::ThrottleGuard;
 use obikseq::CanonicalKmer;
-use obilayeredmap::MphfLayer;
+use obilayeredmap::{layer_dir, MphfLayer};
 use obilayeredmap::meta::PartitionMeta;
 use obisys::progress_bar;
 
@@ -101,8 +101,7 @@ impl SiblingAnnexBuildExt for KmerIndex {
 
             let mut part_slots: u64 = 0;
             for l in 0..meta.n_layers {
-                let layer_dir = index_dir.join(format!("layer_{l}"));
-                part_slots += build_layer_sibling_annex(self, &layer_dir, n_parts, l, &cache)?;
+                part_slots += build_layer_sibling_annex(self, &layer_dir(&index_dir, l), n_parts, l, &cache)?;
             }
             total_slots += part_slots;
             pb.inc(1);