Implement VFS Deduplication (block-level)

- Add DedupStore with content-addressable storage:
  - SHA-256 hash-based block storage
  - Reference counting for block lifecycle
  - dedup_file() and restore_file() operations
  - DedupManifest for file reconstruction
  - DedupStats for storage statistics
- Add VfsDedupConfig:
  - block_size (default 4KB)
  - min_file_size threshold
  - store_path for dedup directory
- Add hex crate for hash encoding
- Block-level dedup foundation for SMB/ZFS

All 229 tests pass.

markbase-core/src/vfs/mod.rs

@@ -1,4 +1,5 @@
 pub mod compression;
+pub mod dedup;
 pub mod local_fs;
 pub mod open_flags;
 pub mod s3_fs;
@@ -430,3 +431,24 @@ pub struct VfsCompressionConfig {
     /// 最小压缩大小（字节），小于此大小不压缩
     pub min_size: u64,
 }
+
+/// 去重配置
+#[derive(Debug, Clone)]
+pub struct VfsDedupConfig {
+    /// 块大小（字节），默认4KB
+    pub block_size: usize,
+    /// 最小文件大小（字节），小于此大小不去重
+    pub min_file_size: u64,
+    /// 去重存储路径
+    pub store_path: PathBuf,
+}
+
+impl Default for VfsDedupConfig {
+    fn default() -> Self {
+        Self {
+            block_size: 4096,
+            min_file_size: 1024,
+            store_path: PathBuf::from(".dedup"),
+        }
+    }
+}