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/src/vfs/mod.rs

@@ -1,3 +1,4 @@
+pub mod compression;
 pub mod local_fs;
 pub mod open_flags;
 pub mod s3_fs;
@@ -259,3 +260,25 @@ pub struct VfsQuotaUsage {
     /// 是否超过硬限制
     pub over_hard_limit: bool,
 }
+
+/// 压缩算法类型
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum VfsCompression {
+    /// 无压缩
+    None,
+    /// LZ4压缩（快速）
+    Lz4,
+    /// ZSTD压缩（高压缩率）
+    Zstd,
+}
+
+/// 压缩配置
+#[derive(Debug, Clone)]
+pub struct VfsCompressionConfig {
+    /// 压缩算法
+    pub algorithm: VfsCompression,
+    /// 压缩级别（1-22 for ZSTD, 1-12 for LZ4）
+    pub level: u32,
+    /// 最小压缩大小（字节），小于此大小不压缩
+    pub min_size: u64,
+}