Implement SMB Previous versions (shadow copy) at VFS layer

- Add VfsPreviousVersion struct (snapshot_name, gmt_token, created, size)
- Add VfsBackend methods:
  - list_previous_versions() - enumerate snapshot versions
  - open_previous_version() - open file from snapshot by GMT token
  - restore_previous_version() - restore file from snapshot
- LocalFs implementation:
  - systemtime_to_gmt_token() - convert SystemTime to @GMT-YYYY.MM.DD-HH.MM.SS
  - scan .snapshots directory for matching versions
  - use existing restore_snapshot() for restoration
- Foundation for SMB shadow copy (@GMT- token support)

All 229 tests pass.

markbase-core/src/vfs/mod.rs

@@ -218,6 +218,23 @@ pub trait VfsBackend: Send + Sync {
     fn check_quota(&self, _path: &Path, _size: u64) -> Result<bool, VfsError> {
         Ok(true) // Default: no quota, always allow
     }
+
+    // ===== Previous versions (shadow copy) =====
+
+    /// 列出文件的所有历史版本
+    fn list_previous_versions(&self, _path: &Path) -> Result<Vec<VfsPreviousVersion>, VfsError> {
+        Err(VfsError::Unsupported("list_previous_versions".to_string()))
+    }
+
+    /// 打开历史版本文件（通过 @GMT- token）
+    fn open_previous_version(&self, _path: &Path, _gmt_token: &str) -> Result<Box<dyn VfsFile>, VfsError> {
+        Err(VfsError::Unsupported("open_previous_version".to_string()))
+    }
+
+    /// 从历史版本恢复文件
+    fn restore_previous_version(&self, _path: &Path, _gmt_token: &str) -> Result<(), VfsError> {
+        Err(VfsError::Unsupported("restore_previous_version".to_string()))
+    }
 }
 
 /// 快照信息
@@ -261,6 +278,19 @@ pub struct VfsQuotaUsage {
     pub over_hard_limit: bool,
 }
 
+/// 历史版本信息（SMB shadow copy）
+#[derive(Debug, Clone)]
+pub struct VfsPreviousVersion {
+    /// 快照名称
+    pub snapshot_name: String,
+    /// GMT token (@GMT-YYYY.MM.DD-HH.MM.SS)
+    pub gmt_token: String,
+    /// 创建时间
+    pub created: SystemTime,
+    /// 版本大小（字节）
+    pub size: u64,
+}
+
 /// 压缩算法类型
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub enum VfsCompression {