feat: introduce unified column view types for bit and int matrices

This commit introduces `BitColView` and `IntColView` to abstract over Columnar and Packed storage formats, implementing `BitSlice` and `IntSlice` for uniform column access. It adds `col_view()` accessors to `PersistentBitMatrix` and `PackedCompactIntMatrix`, explicitly panicking on implicit variants. The new types are publicly re-exported, and unit tests are added to validate per-element retrieval, aggregation methods, and parity with the original columnar representation.

src/obicompactvec/src/intmatrix.rs

@@ -224,6 +224,43 @@ impl Iterator for PackedIntColIter<'_> {
 
 impl ExactSizeIterator for PackedIntColIter<'_> {}
 
+// ── IntColView — uniform column access across Columnar and Packed ─────────────
+
+enum IntColViewInner<'a> {
+    Columnar(&'a PersistentCompactIntVec),
+    Packed(PackedIntCol<'a>),
+}
+
+/// Opaque column view returned by [`PersistentCompactIntMatrix::col_view`].
+/// Implements [`IntSlice`] uniformly for both Columnar and Packed matrix formats.
+pub struct IntColView<'a>(IntColViewInner<'a>);
+
+impl IntSlice for IntColView<'_> {
+    fn len(&self) -> usize {
+        match &self.0 { IntColViewInner::Columnar(c) => c.len(), IntColViewInner::Packed(c) => c.len() }
+    }
+    fn get(&self, slot: usize) -> u32 {
+        match &self.0 { IntColViewInner::Columnar(c) => c.get(slot), IntColViewInner::Packed(c) => c.get(slot) }
+    }
+    fn primary_bytes(&self) -> &[u8] {
+        match &self.0 { IntColViewInner::Columnar(c) => c.primary_bytes(), IntColViewInner::Packed(c) => c.primary_bytes() }
+    }
+    fn overflow_entries(&self) -> impl Iterator<Item = (usize, u32)> + '_ {
+        // Box<dyn Iterator> implements Iterator, satisfying RPITIT across two distinct types.
+        let it: Box<dyn Iterator<Item = (usize, u32)> + '_> = match &self.0 {
+            IntColViewInner::Columnar(c) => Box::new(c.overflow_entries()),
+            IntColViewInner::Packed(c)   => Box::new(c.overflow_entries()),
+        };
+        it
+    }
+    fn sum(&self) -> u64 {
+        match &self.0 { IntColViewInner::Columnar(c) => c.sum(), IntColViewInner::Packed(c) => c.sum() }
+    }
+    fn count_nonzero(&self) -> u64 {
+        match &self.0 { IntColViewInner::Columnar(c) => c.count_nonzero(), IntColViewInner::Packed(c) => c.count_nonzero() }
+    }
+}
+
 // ─────────────────────────────────────────────────────────────────────────────
 
 pub struct PackedCompactIntMatrix {
@@ -481,6 +518,13 @@ impl PersistentCompactIntMatrix {
         }
     }
 
+    pub fn col_view(&self, c: usize) -> IntColView<'_> {
+        match self {
+            Self::Columnar(m) => IntColView(IntColViewInner::Columnar(m.col(c))),
+            Self::Packed(m)   => IntColView(IntColViewInner::Packed(m.col_slice(c))),
+        }
+    }
+
     pub fn col_persist(&self, c: usize, path: &Path) -> io::Result<PersistentCompactIntVecBuilder> {
         match self {
             Self::Columnar(m) => PersistentCompactIntVecBuilder::build_from(m.col(c), path),