saprse_set: deprecate .at as ambiguous

src/entt/entity/sparse_set.hpp

@@ -754,7 +754,7 @@ public:
      * @param pos The position for which to return the entity.
      * @return The entity at specified location if any, a null entity otherwise.
      */
-    [[nodiscard]] entity_type at(const size_type pos) const noexcept {
+    [[deprecated("use .begin()[pos] instead")]] [[nodiscard]] entity_type at(const size_type pos) const noexcept {
         return pos < packed.size() ? packed[pos] : null;
     }