Design a Thread-Safe Concurrent LRU Cache

Question

## Problem

Implement an LRU cache that is correct under concurrent `get`/`put` from many threads (Snowflake's concurrency round).

## Example
```
Multiple threads; O(1) ops; no races
```

## Constraints
- High contention

## Discussion
Lock striping/sharding vs a single lock; trade-offs vs a lock-free design. Snowflake prefers correct-and-reasoned over clever-but-fragile.