LRU Cache

Implement `LRUCache(capacity)` with `get(key)` returning the value or `-1`, and `put(key, value)`. When the cache is full, evict the least recently used entry. Both operations must be O(1). Neither structure alone is enough: a map gives O(1) lookup but no ordering, a list gives ordering but O(n) lookup. Combine them — the map stores key to node, the doubly linked list holds recency order, and a `get` counts as a use.