Zstandard State Caching

For server and reader applications, re-parsing large books on every request creates unnecessary CPU overhead. ebook-rs supports zero-overhead snapshot caching using Zstandard.

// Save compressed book state to disk / Redis
let cache_bytes = book.export_zstd_cache()?;

// Instantly restore in <1ms without re-parsing
let restored_book = Book::from_zstd_cache(&cache_bytes)?;
```\n