Kth Largest in a Stream

Values arrive one at a time and never stop. After each arrival, report the kth largest value seen so far. Implement a `KthLargest` class: - `KthLargest(k)` — construct for a fixed k. - `add(value)` — record a value and return the current kth largest, or `None` if fewer than k values have been seen. You cannot store the whole stream. Memory must depend on `k`, not on how long the stream runs — and be ready to explain why the heap you reach for holds the *smallest* of the values you care about.