feat: add col_weights API and refactor obikstats to use Algorithm trait

Added a `col_weights` method to index layers for computing per-genome column sums or presence k-mer counts. Refactored `obikstats` to implement the `Algorithm` trait with a two-phase `new`/`run` model, replacing manual layer resolution with `IndexCache` for eager file I/O. Simplified per-genome counting logic and updated public exports and dependencies accordingly.

src/obikindex/src/layer/content_layer.rs

@@ -217,6 +217,16 @@ impl KmerLayer {
         }
     }
 
+    /// Per-genome column weights — count sum or presence k-mer count,
+    /// depending on content (see `obicompactvec::ColumnWeights`).
+    pub fn col_weights(&self) -> ndarray::Array1<u64> {
+        match self {
+            KmerLayer::Count { layer, .. } => layer.col_weights(),
+            KmerLayer::Presence { layer, .. } => layer.col_weights(),
+            KmerLayer::Empty { .. } => panic!("Layer::col_weights() called on an Empty layer"),
+        }
+    }
+
     /// Batch, genome-major "carries" for a set of `slots` — `out[g][i]` =
     /// whether genome `g` (0..`out.len()`) carries `slots[i]`. `out` must
     /// have one entry per genome column, each resized to `slots.len()`.

src/obikindex/src/layer/typed_layer.rs

@@ -364,6 +364,12 @@ impl TypedLayer<PersistentCompactIntMatrix> {
     pub fn fill_sub_matrix(&self, slots: &[usize], out: &mut [Vec<u32>]) {
         self.data.fill_sub_matrix(slots, out)
     }
+
+    /// Per-genome column weights — for counts, the sum of values in each
+    /// column (see `obicompactvec::ColumnWeights::col_weights`).
+    pub fn col_weights(&self) -> ndarray::Array1<u64> {
+        obicompactvec::ColumnWeights::col_weights(&self.data)
+    }
 }
 
 // ── Mode 3 — presence/absence matrix ─────────────────────────────────────────
@@ -379,7 +385,7 @@ impl TypedLayer<PersistentCompactIntMatrix> {
 // below: sparse matrices aren't built column-by-column, they're built
 // row-by-row from an already-built dense layer
 // (`PersistentSparseBitMatrixBuilder::build_from_dense`).
-impl<D: LayerData<Item = Box<[bool]>> + BinaryMatrix> TypedLayer<D> {
+impl<D: LayerData<Item = Box<[bool]>> + BinaryMatrix + obicompactvec::ColumnWeights> TypedLayer<D> {
     /// Number of genome columns in this layer's presence matrix — see
     /// `PersistentBitMatrix::n_cols`'s docs for the `Implicit` mono-genome
     /// special case (always reports `1`, regardless of the index's real
@@ -408,6 +414,13 @@ impl<D: LayerData<Item = Box<[bool]>> + BinaryMatrix> TypedLayer<D> {
     pub fn fill_sub_matrix(&self, slots: &[usize], out: &mut [Vec<bool>]) {
         self.data.fill_sub_matrix(slots, out)
     }
+
+    /// Per-genome column weights — for presence/absence, the number of
+    /// k-mers each genome carries (see
+    /// `obicompactvec::ColumnWeights::col_weights`).
+    pub fn col_weights(&self) -> ndarray::Array1<u64> {
+        obicompactvec::ColumnWeights::col_weights(&self.data)
+    }
 }
 
 impl TypedLayer<PersistentBitMatrix> {