feat: implement per-k-mer sibling counts and central neighbor generation

Introduce the siblingannex module in obicompactvec to store per-slot minorant flags and sibling counts in a memory-mapped annex file. Add a scatter-gather pipeline in obikindex to compute these values across index layers and write them to .psib files. Implement central_canonical_neighbors in obikseq for generating strand-aware k-mer variants around the middle base. Expose rolling statistics in obiskbuilder and update dependency graphs accordingly.

src/obikseq/src/tests/kmer.rs

@@ -210,4 +210,46 @@ mod tests {
         check!(31);
         check!(32);
     }
+
+    // ── central_canonical_neighbors ─────────────────────────────────────────
+
+    #[test]
+    fn central_canonical_neighbors_hand_checked_k3() {
+        // k=3, centre = index 1. For "ACG", every one of the 4 central
+        // substitutions ("AAG","ACG","AGG","ATG") happens to stay in forward
+        // orientation when canonicalised (verified by hand: each is already
+        // lexicographically <= its own reverse complement), so this case
+        // exercises the substitution logic without the RC-flip edge case.
+        let ck = KmerOf::<ConstLen<3>>::from_ascii(b"ACG").unwrap().canonical();
+        let neighbours = ck.central_canonical_neighbors();
+        let ascii: Vec<Vec<u8>> = neighbours.iter().map(|n| n.to_ascii()).collect();
+        assert_eq!(ascii, vec![b"AAG".to_vec(), b"ACG".to_vec(), b"AGG".to_vec(), b"ATG".to_vec()]);
+        // The identity substitution (centre unchanged) must reproduce `ck`.
+        assert!(neighbours.contains(&ck));
+    }
+
+    #[test]
+    fn central_canonical_neighbors_identity_present_for_various_k() {
+        macro_rules! check {
+            ($n:expr) => {{
+                let ck = KmerOf::<ConstLen<$n>>::from_ascii(&make_seq::<$n>())
+                    .unwrap()
+                    .canonical();
+                let neighbours = ck.central_canonical_neighbors();
+                assert!(
+                    neighbours.contains(&ck),
+                    "identity substitution missing from central_canonical_neighbors for k={}",
+                    $n
+                );
+                // Every returned neighbour must itself already be canonical.
+                for n in &neighbours {
+                    assert_eq!(n.into_kmer().canonical(), *n, "neighbour not canonical for k={}", $n);
+                }
+            }};
+        }
+        check!(1);
+        check!(3);
+        check!(5);
+        check!(31);
+    }
 }