rzstd 0.2.0

Ruby bindings (via Rust/magnus) for the Zstandard compressor with persistent ZSTD_CCtx / ZSTD_DCtx contexts that are reused across calls. Provides Zstd frame compress/decompress at module level and a stateful Dictionary class for dict-bound compression. Designed to be safe to call from multiple Ractors and competitive with rlz4 on small messages, where per-call context allocation in zstd-ruby dominates the cost.