Add layout-aware drop targeting for custom layouts (#605)

_Describe your changes here. Please include screenshots if they're
visual!_

This change allows custom ListLayout implementations to override
drop target determination during interactive item reordering.

- Add targetIndexPath(forInteractivelyMovingItem:withPosition:) protocol
method to ListLayout with default nil implementation
- Add CollectionViewLayout override that delegates to custom layout
- Add isReorderable property on ListLayoutContent.ItemInfo to expose
whether an item has reordering enabled

### Checklist

Please do the following before merging:

- [x] Ensure any public-facing changes are reflected in the
[changelog](https://github.com/square/Listable/blob/main/CHANGELOG.md).
Include them in the `Main` section.

Author: Gabriel Hernández Ontiveros

CHANGELOG.md

@@ -4,6 +4,9 @@
 
 ### Added
 
+- Added `targetIndexPath(forInteractivelyMovingItem:withPosition:)` protocol method to `ListLayout`, allowing custom layouts to override drop target determination during interactive reordering.
+- Added `isReorderable` property on `ListLayoutContent.ItemInfo` to check if an item has reordering enabled.
+
 ### Removed
 
 ### Changed

ListableUI/Sources/Layout/CollectionViewLayout.swift

@@ -623,9 +623,15 @@ final class CollectionViewLayout : UICollectionViewLayout
         withPosition position: CGPoint
     ) -> IndexPath {
 
-        /// TODO: The default implementation provided by `UICollectionView` does not work correctly
-        /// when trying to move an item to the end of a section, or when trying to move an item into an
-        /// empty section. We should add casing that allows moving into the section in these cases.
+        // Allow custom layouts to provide layout-aware drop targeting.
+        // This fixes issues with UICollectionView's default implementation which doesn't
+        // work correctly for some layout types.
+        if let customTarget = self.layout.targetIndexPath(
+            forInteractivelyMovingItem: previousIndexPath,
+            withPosition: position
+        ) {
+            return customTarget
+        }
 
         return super.targetIndexPath(forInteractivelyMovingItem: previousIndexPath, withPosition: position)
     }

ListableUI/Sources/Layout/ListLayout/ListLayout.swift

@@ -168,6 +168,21 @@ public protocol AnyListLayout : AnyObject
         at indexPath: IndexPath,
         withTargetPosition position: CGPoint
     )
+
+    /// Returns the target index path for an item being interactively moved.
+    ///
+    /// Custom layouts can override this to provide layout-aware drop target
+    /// determination. The default implementation returns `nil`, which causes
+    /// `CollectionViewLayout` to fall back to UICollectionView's default behavior.
+    ///
+    /// - Parameters:
+    ///   - previousIndexPath: The current index path of the item being moved.
+    ///   - position: The current position of the item in the collection view's coordinate space.
+    /// - Returns: The target index path if the layout can determine it, or `nil` to use default behavior.
+    func targetIndexPath(
+        forInteractivelyMovingItem previousIndexPath: IndexPath,
+        withPosition position: CGPoint
+    ) -> IndexPath?
 }
 
 
@@ -338,6 +353,15 @@ extension ListLayout
     ) {
         // Nothing. Just a default implementation.
     }
+
+    public func targetIndexPath(
+        forInteractivelyMovingItem previousIndexPath: IndexPath,
+        withPosition position: CGPoint
+    ) -> IndexPath? {
+        // Default: return nil to use UICollectionView's default behavior.
+        // Custom layouts can override this for layout-aware drop targeting.
+        nil
+    }
 
     private static func isHeaderSticky(
         list: Bool,

ListableUI/Sources/Layout/ListLayout/ListLayoutContent.swift

@@ -510,6 +510,11 @@ extension ListLayoutContent
         public var layouts : ItemLayouts {
             self.state.anyModel.layouts
         }
+
+        /// Whether this item can be reordered (has reordering configuration).
+        public var isReorderable: Bool {
+            self.state.anyModel.reordering != nil
+        }
 
         public var frame : CGRect {
             CGRect(