feat(data-structures): add LRU cache

[0xDevNinja]

Summary

• Adds LruCache<K, V> in src/data_structures/lru_cache.rs with new, get, put, len, is_empty, capacity, contains_key.
• O(1) amortized get/put. Implemented with a Vec-backed slab of doubly-linked nodes (indices, not pointers) plus a HashMap<K, usize> for index lookup. Most-recently-used at the head, least-recently-used at the tail. Eviction recycles slab slots via a free-list. Entirely safe Rust.
• capacity == 0 is documented and tested: every put returns the just-inserted value as evicted; the cache stays empty.

Test plan

• cargo fmt --check
• cargo clippy --all-targets -- -D warnings
• cargo test (318 tests pass)
• Unit tests cover empty cache, capacity-0, capacity-1 (newest only), basic put+get, LRU eviction order over 4 inserts into a 3-item cache, get promoting an entry to MRU, updating an existing key bumping it to MRU without eviction, get of an absent key returning None, and len / is_empty / capacity correctness.
• quickcheck property test compares the implementation against a Vec<(K, V)>-backed brute-force LRU oracle (capacity 0..=8, ops capped at 50), checking get results, the live-key set, and stored values.

Closes #67