perf: optimize packed_seq sub() with direct bit-slice copying

Replaces per-nucleotide iteration with direct bit-slice copying via `bitvec`. This eliminates per-element decoding overhead and intermediate allocations by computing the target byte length, copying the packed bit range `[start*2, end*2)` directly into a pre-allocated buffer, and constructing the result in a single pass.

src/obikseq/src/packed_seq.rs

@@ -261,8 +261,13 @@ impl PackedSeq {
     /// Extract nucleotides `[start, end)` as a new [`PackedSeq`]. Allocates.
     pub fn sub(&self, start: usize, end: usize) -> Self {
         debug_assert!(end > start && end <= self.seql());
-        let nucs: Vec<u8> = (start..end).map(|i| self.nucleotide(i)).collect();
-        Self::from_nucleotides(&nucs)
+        let seql = end - start;
+        let n = (seql + 3) / 4;
+        let mut out = vec![0u8; n];
+        let src = self.seq.view_bits::<Msb0>();
+        let dst = out.view_bits_mut::<Msb0>();
+        dst[..seql * 2].copy_from_bitslice(&src[start * 2..end * 2]);
+        Self::new(count_to_tail(seql), out.into_boxed_slice())
     }
 
     /// Serialise one chunk to binary.