LRU Cache

Implement class LRUCache with:

• constructor(capacity: number)
• get(key) → value or undefined; promotes the entry to most-recent
• set(key, value) → adds or updates; evicts the least-recently-used when over capacity

Both ops must be O(1).

Hint: JS Map preserves insertion order. Use that.

Analyze time + space; defend the O(1) claim.