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/obiskbuilder/src/lib.rs

@@ -10,7 +10,8 @@ pub mod stream_iter;
 mod scratch;
 
 pub(crate) mod encoding;
-pub(crate) mod rolling_stat;
+#[allow(missing_docs)]
+pub mod rolling_stat;
 
 pub use iter::SuperKmerIter;
 pub use scratch::SuperKmerScratch;