refactor: unify iterator construction in nonzero_iter

Change the return type of `nonzero_iter` from an opaque `impl Iterator` to a `Box<dyn Iterator>`. This enables direct return coercion in the `Count` layer branch, removing an explicit `Box::new()` wrapper and aligning it with the existing `Presence` implementation. The modification shifts iterator construction to runtime dynamic dispatch while preserving the public API contract and iteration semantics.

src/obikindex/src/layer/content_layer.rs

@@ -224,7 +224,7 @@ impl KmerLayer {
     pub fn nonzero_iter<'a>(&'a self, slots: &'a [usize]) -> Box<dyn Iterator<Item = (usize, usize, u32)> + 'a> {
         match self {
             KmerLayer::Presence { layer, .. } => layer.nonzero_iter(slots),
-            KmerLayer::Count { layer, .. } => Box::new(layer.nonzero_iter(slots)),
+            KmerLayer::Count { layer, .. } => layer.nonzero_iter(slots),
             KmerLayer::Empty { .. } => panic!("Layer::nonzero_iter() called on an Empty layer"),
         }
     }

src/obikindex/src/layer/typed_layer.rs

@@ -418,7 +418,7 @@ impl TypedLayer<PersistentCompactIntMatrix> {
 
     /// Every nonzero `(idx into slots, col, value)` triple among `slots` —
     /// delegates to `PersistentCompactIntMatrix::nonzero_iter`.
-    pub fn nonzero_iter<'a>(&'a self, slots: &'a [usize]) -> impl Iterator<Item = (usize, usize, u32)> + 'a {
+    pub fn nonzero_iter<'a>(&'a self, slots: &'a [usize]) -> Box<dyn Iterator<Item = (usize, usize, u32)> + 'a> {
         self.data.nonzero_iter(slots)
     }
 }