Implement VFS compression support (ZSTD)

- Add VfsCompression and VfsCompressionConfig types
- Add compression module with Compressor:
  - compress/decompress methods
  - compress_file/decompress_file utilities
  - should_compress threshold check
  - extension detection (.zst, .lz4)
- Add zstd crate dependency
- LZ4 placeholder (future implementation)

Enables SMB transparent compression.

All 229 tests pass.

markbase-core/Cargo.toml

@@ -47,6 +47,7 @@ ssh-key = "0.7.0-rc.10"
 rand = "0.8"
 axum-extra = { version = "0.9", features = ["multipart"] }
 tokio-util = { version = "0.7", features = ["io"] }
+zstd = "0.13"
 toml = "0.8"
 uuid = { version = "1", features = ["v4"] }
 dashmap = "6.1"