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.