refactor: optimize unitig iteration and graph traversal

Switches unitig processing to a lazy, fallible `try_for_each_unitig` API across partitioner layers, reducing intermediate allocations and enabling proper error propagation. Refactors de Bruijn graph traversal into a two-pass algorithm with explicit node flags, named constants, and diagnostic logging. Introduces parallel chain processing and staged performance profiling for the unitig command, and adds a memory-efficient `FromIterator` implementation for packed nucleotide sequences.

src/obikpartitionner/src/index_layer.rs

@@ -104,9 +104,10 @@ impl KmerPartition {
         fs::create_dir_all(&layer_dir)?;
 
         let mut uw = Layer::<()>::unitig_writer(&layer_dir).map_err(olm_to_sk)?;
-        for unitig in g.iter_unitig() {
-            uw.write(&unitig)?;
-        }
+        g.try_for_each_unitig(|nuc_iter| {
+            let unitig: obikseq::unitig::Unitig = nuc_iter.collect();
+            uw.write(&unitig)
+        })?;
         uw.close()?;
 
         if with_counts {

src/obikpartitionner/src/merge_layer.rs

@@ -202,9 +202,10 @@ impl KmerPartition {
             g.compute_degrees();
             fs::create_dir_all(&new_layer_dir)?;
             let mut uw = Layer::<()>::unitig_writer(&new_layer_dir).map_err(olm_to_sk)?;
-            for unitig in g.iter_unitig() {
-                uw.write(&unitig)?;
-            }
+            g.try_for_each_unitig(|nuc_iter| {
+                let unitig: obikseq::unitig::Unitig = nuc_iter.collect();
+                uw.write(&unitig)
+            })?;
             uw.close()?;
             Layer::<()>::build(&new_layer_dir, block_bits, evidence).map_err(olm_to_sk)?;
             g.len()

src/obikpartitionner/src/rebuild_layer.rs

@@ -154,9 +154,10 @@ impl KmerPartition {
         fs::create_dir_all(&dst_layer_dir)?;
 
         let mut uw = Layer::<()>::unitig_writer(&dst_layer_dir).map_err(olm_to_sk)?;
-        for unitig in g.iter_unitig() {
-            uw.write(&unitig)?;
-        }
+        g.try_for_each_unitig(|nuc_iter| {
+            let unitig: obikseq::unitig::Unitig = nuc_iter.collect();
+            uw.write(&unitig)
+        })?;
         uw.close()?;
         drop(g);