Implement LRU object cache for deduplication performance

[UtsavBalar1231]

Add an optional LRU (Least Recently Used) cache for frequently accessed objects to improve performance during operations involving many files.

Context

Currently in src/config/mod.rs:176, there's a commented-out placeholder for future cache configuration:

// TODO: Add cache_size field when implementing object caching
// pub cache_size: usize,  // Reserved for future LRU cache implementation

Proposed Changes

1. Add cache_size field to CoreConfig with sensible default (e.g., 1000 objects)
2. Implement LRU cache in src/storage/mod.rs using a library like lru crate
3. Cache frequently accessed file hashes and metadata
4. Add benchmarks to measure performance improvement
5. Document cache behavior in user-facing docs

Benefits

• Faster repeated operations on same files (e.g., multiple status checks)
• Reduced disk I/O for recently accessed objects
• Configurable memory/performance tradeoff

Priority: Low (optimization)