shared_ptr lifetime counters

Implement struct SharedProbe with a static alive() counter (constructor increments, destructor decrements) and the free function int probe_lifetimes() that: creates a shared_ptr<SharedProbe>, copies it, lets the copy go out of scope in an inner block, then resets the original — returning alive() measured after all of that. Correct code returns 0; the test proves both the copy and the reset released.

Difficulty: intermediate