refactor: replace explicit collect with Unitig::from_nucleotides

Introduce a thread-local buffer to materialize nucleotide iterators into contiguous slices. Update `try_for_each_unitig` across the debruijn, index, merge, and rebuild layers to directly instantiate `Unitig` via `from_nucleotides()` instead of explicitly collecting iterators. This eliminates intermediate allocations and aligns test code with the new approach.

src/obikpartitionner/src/index_layer.rs

@@ -107,9 +107,8 @@ impl KmerPartition {
         fs::create_dir_all(&layer_dir)?;
 
         let mut uw = Layer::<()>::unitig_writer(&layer_dir).map_err(olm_to_sk)?;
-        g.try_for_each_unitig(|nuc_iter| {
-            let unitig: obikseq::unitig::Unitig = nuc_iter.collect();
-            uw.write(&unitig)
+        g.try_for_each_unitig(|nucs| {
+            uw.write(&obikseq::unitig::Unitig::from_nucleotides(nucs))
         })?;
         uw.close()?;
 

src/obikpartitionner/src/merge_layer.rs

@@ -307,9 +307,8 @@ impl KmerPartition {
             g.compute_degrees_and_mark_starts();
             fs::create_dir_all(&new_layer_dir)?;
             let mut uw = Layer::<()>::unitig_writer(&new_layer_dir).map_err(olm_to_sk)?;
-            g.try_for_each_unitig(|nuc_iter| {
-                let unitig: obikseq::unitig::Unitig = nuc_iter.collect();
-                uw.write(&unitig)
+            g.try_for_each_unitig(|nucs| {
+                uw.write(&obikseq::unitig::Unitig::from_nucleotides(nucs))
             })?;
             uw.close()?;
             let n = g.len();

src/obikpartitionner/src/rebuild_layer.rs

@@ -168,9 +168,8 @@ impl KmerPartition {
         fs::create_dir_all(&dst_layer_dir)?;
 
         let mut uw = Layer::<()>::unitig_writer(&dst_layer_dir).map_err(olm_to_sk)?;
-        g.try_for_each_unitig(|nuc_iter| {
-            let unitig: obikseq::unitig::Unitig = nuc_iter.collect();
-            uw.write(&unitig)
+        g.try_for_each_unitig(|nucs| {
+            uw.write(&obikseq::unitig::Unitig::from_nucleotides(nucs))
         })?;
         uw.close()?;
         drop(g);