Implement a reference-counted smart pointer

Implement a minimal C++ reference-counted smart pointer similar to `std::shared_ptr`. Requirements: - Create a class template `SharedPtr<T>` that manages a heap-allocated `T*`. - Use RAII: the managed object must be deleted exactly once when the last `SharedPtr` owning it is destroyed. - Support: - Default constructor (null pointer) - Constructor from raw pointer `T*` - Destructor - Copy constructor / copy assignment (increase reference count) - Move constructor / move assignment (transfer ownership, leaving the source null) - `T& operator*()`, `T* operator->()`, `T* get() const` - `size_t use_count() const` - `void reset(T* p = nullptr)` - Handle self-assignment correctly and avoid memory leaks/double-free. You may ignore thread-safety unless you explicitly choose to support it.