Build a cache library with pluggable eviction and TTL

• A Cache with get(key), set(key, value, { ttlMs }?), delete(key), has(key), clear() and size, with a fixed capacity
• Eviction policy is pluggable through an interface; implement LRU (least recently used) and LFU (least frequently used, with LRU as tie-break), both with O(1) get and set
• TTL: entries expire after their time to live; an expired entry is never returned and does not count towards the size; expiry is enforced lazily on access plus a periodic sweep
• When the capacity is exceeded, the policy picks the victim; updating an existing key does not evict anything
• Statistics: hits, misses, evictions, expirations, and the hit ratio
• Time is injectable through a clock so TTL behaviour is testable without sleeping
• Thread-safety note: operations are synchronous and therefore atomic in Node; for an async loader provide getOrLoad(key, loader) that deduplicates concurrent loads of the same key

Only after every requirement works.

• Per-entry size limit (a max total weight instead of count)
• Write-through to a backing store, and refresh-ahead for hot keys
• Event hooks for eviction and expiration