Thread-Safe Counter

Implement a `ThreadSafeCounter` class with two methods: - `increment()` — increments the counter by 1. - `value()` — returns the current count. Multiple threads will call `increment()` concurrently. The final count after all increments complete must be exactly correct — no lost updates. This is the single most-asked "build the primitive" question in real interviews, because it's the smallest possible example of a race condition: two threads can both read the same value before either writes back the incremented result, silently dropping an increment.