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/obikseq/src/packed_seq.rs

@@ -115,6 +115,29 @@ impl PackedSeq {
         }
         Self::new(count_to_tail(seql), seq.into_boxed_slice())
     }
+}
+
+impl FromIterator<u8> for PackedSeq {
+    /// Build from a stream of 2-bit nucleotide values (0=A…3=T).
+    /// Packs on the fly — one allocation, no intermediate Vec.
+    fn from_iter<I: IntoIterator<Item = u8>>(iter: I) -> Self {
+        let iter = iter.into_iter();
+        let (lower, _) = iter.size_hint();
+        let mut seq: Vec<u8> = Vec::with_capacity(byte_len(lower.max(1)));
+        let mut seql = 0usize;
+        for nuc in iter {
+            if seql % 4 == 0 {
+                seq.push(0);
+            }
+            *seq.last_mut().unwrap() |= (nuc & 0b11) << (6 - 2 * (seql % 4));
+            seql += 1;
+        }
+        debug_assert!(seql >= 1, "PackedSeq requires at least one nucleotide");
+        Self::new(count_to_tail(seql), seq.into_boxed_slice())
+    }
+}
+
+impl PackedSeq {
 
     /// Write ASCII nucleotides into `writer`. Zero allocation.
     pub fn write_ascii<W: Write>(&self, writer: &mut W) -> io::Result<()> {