feat(matrix): add partial group reductions and column persistence

Expands MatrixGroupOps with partial_group_min/max helpers for bitwise reductions and introduces add_col_from methods to persist external vectors as matrix columns. Refactors column aggregation in the partitioner to leverage these group operations directly, replacing iterative row processing with simplified builder lifecycle management and explicit metadata serialization.

src/obicompactvec/src/intmatrix.rs

@@ -386,6 +386,21 @@ impl PersistentCompactIntMatrixBuilder {
         self.n_cols += 1;
         PersistentCompactIntVecBuilder::new(self.n, &path)
     }
+
+    pub fn add_col_from(&mut self, src: &TempCompactIntVec) -> io::Result<()> {
+        src.make_persistent(&col_path(&self.dir, self.n_cols))?;
+        self.n_cols += 1;
+        Ok(())
+    }
+
+    pub fn add_col_from_bit(&mut self, src: &TempBitVec) -> io::Result<()> {
+        let path = col_path(&self.dir, self.n_cols);
+        self.n_cols += 1;
+        let mut b = PersistentCompactIntVecBuilder::new(self.n, &path)?;
+        b.inc_present(src.view());
+        b.close()
+    }
+
     pub fn close(self) -> io::Result<()> {
         MatrixMeta { n: self.n, n_cols: self.n_cols }.save(&self.dir)
     }
@@ -431,4 +446,21 @@ impl MatrixGroupOps for PersistentCompactIntMatrix {
         }
         result.freeze()
     }
+
+    fn partial_group_min(&self, g: &ColGroup) -> io::Result<TempCompactIntVec> {
+        let n = self.n();
+        let mut result = TempCompactIntVecBuilder::new(n)?;
+        if let Some((&first, rest)) = g.indices.split_first() {
+            result.add(self.col_view(first));
+            for &c in rest { result.min(self.col_view(c)); }
+        }
+        result.freeze()
+    }
+
+    fn partial_group_max(&self, g: &ColGroup) -> io::Result<TempCompactIntVec> {
+        let n = self.n();
+        let mut result = TempCompactIntVecBuilder::new(n)?;
+        for &c in &g.indices { result.max(self.col_view(c)); }
+        result.freeze()
+    }
 }