Implement VFS snapshot support (ZFS-style)

- Add VfsSnapshotInfo struct
- Add snapshot methods to VfsBackend trait:
  - create_snapshot: copy-on-write with metadata
  - list_snapshots: enumerate snapshots
  - delete_snapshot: remove snapshot and metadata
  - restore_snapshot: restore from snapshot
  - snapshot_info: get snapshot metadata
- Implement LocalFs snapshot support:
  - Uses .snapshots directory for storage
  - JSON metadata files (*.meta)
  - Recursive directory copy
  - Size calculation

This enables SMB 'Previous versions' feature foundation.

All 229 tests pass.

markbase-core/src/vfs/mod.rs

@@ -168,4 +168,44 @@ pub trait VfsBackend: Send + Sync {
 
     /// 创建硬链接
     fn hard_link(&self, original: &Path, link: &Path) -> Result<(), VfsError>;
+
+    // ===== Snapshot support (ZFS-style) =====
+
+    /// 创建快照
+    fn create_snapshot(&self, _path: &Path, _name: &str) -> Result<(), VfsError> {
+        Err(VfsError::Unsupported("create_snapshot".to_string()))
+    }
+
+    /// 列出快照
+    fn list_snapshots(&self, _path: &Path) -> Result<Vec<String>, VfsError> {
+        Err(VfsError::Unsupported("list_snapshots".to_string()))
+    }
+
+    /// 删除快照
+    fn delete_snapshot(&self, _path: &Path, _name: &str) -> Result<(), VfsError> {
+        Err(VfsError::Unsupported("delete_snapshot".to_string()))
+    }
+
+    /// 从快照恢复
+    fn restore_snapshot(&self, _path: &Path, _name: &str) -> Result<(), VfsError> {
+        Err(VfsError::Unsupported("restore_snapshot".to_string()))
+    }
+
+    /// 获取快照信息
+    fn snapshot_info(&self, _path: &Path, _name: &str) -> Result<VfsSnapshotInfo, VfsError> {
+        Err(VfsError::Unsupported("snapshot_info".to_string()))
+    }
+}
+
+/// 快照信息
+#[derive(Debug, Clone)]
+pub struct VfsSnapshotInfo {
+    /// 快照名称
+    pub name: String,
+    /// 创建时间
+    pub created: SystemTime,
+    /// 快照大小（字节）
+    pub size: u64,
+    /// 是否只读
+    pub read_only: bool,
 }