perf: optimize chunk_start for single-block indexing

Bypasses bitwise shift and mask operations when `block_bits == 0`, directly indexing `self.block_offsets[i]` instead. This eliminates unnecessary arithmetic overhead for single-block cases while preserving the original block-based offset calculation for larger block sizes.

src/obiskio/src/unitig_index.rs

@@ -271,6 +271,9 @@ impl UnitigFileReader {
     fn chunk_start(&self, i: usize) -> usize {
         assert!(!self.block_offsets.is_empty(),
             "random access requires UnitigFileReader::open(); use open_sequential() for iteration only");
+        if self.block_bits == 0 {
+            return self.block_offsets[i] as usize;
+        }
         let block = i >> self.block_bits;
         let rem   = i &  self.mask;
         let mut offset = self.block_offsets[block] as usize;