Store minorant flag in family mask to avoid costly k-mer reconstruction

Transition the minorant flag from a derived value to a stored field within the family mask, resolving a performance regression where on-the-fly reconstruction consumed significant query time. This change introduces O(1) k-mer reconstruction APIs, shifts minorant computation to the index build phase, and enables direct annex-based statistics. Supporting updates include adopting shared ownership for partition caches and refactoring batch processing pipelines.

src/obiskio/src/unitig_index/reader.rs

@@ -157,6 +157,16 @@ impl UnitigFileReader {
         canonical_raw(self.raw_kmer(i, j), self.k) == query.raw()
     }
 
+    /// Canonical raw k-mer at position `j` of chunk `i` — [`raw_kmer`](Self::raw_kmer)
+    /// reduced to its canonical (`min(raw, revcomp)`) form. The reconstruction
+    /// half of [`verify_canonical_kmer`](Self::verify_canonical_kmer): that
+    /// method computes the same value internally just to compare it away: use
+    /// this one when the caller wants the k-mer itself, not a yes/no answer.
+    #[inline]
+    pub fn canonical_raw_kmer(&self, i: usize, j: usize) -> u64 {
+        canonical_raw(self.raw_kmer(i, j), self.k)
+    }
+
     // ── Sequential iterators (O(n) running-offset cursor) ─────────────────────
 
     pub(crate) fn iter_chunks_sequential(&self) -> impl Iterator<Item = (usize, Unitig)> + '_ {