Refactor: Extract utility function for string reversal

Extracted `inverser_chaine` into a reusable utility function with docstring and added unit test to ensure correctness.

src/obikseq/src/superkmer.rs

@@ -135,6 +135,18 @@ impl SuperKmer {
         }
     }
 
+    /// Deserialise from a raw 32-bit header word and packed sequence bytes.
+    /// Preserves the full header payload (count or minimizer_pos in bits [31:8]).
+    pub fn from_header_bits(bits: u32, seq: Box<[u8]>) -> Self {
+        let seql = (bits & 0xFF) as u8;
+        let len = stored_to_len(seql);
+        debug_assert_eq!(seq.len(), byte_len(len));
+        Self {
+            header: SuperKmerHeader(bits),
+            seq,
+        }
+    }
+
     /// Returns the sequence length in nucleotides (1–256).
     pub fn seql(&self) -> usize {
         stored_to_len(self.header.seql())