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/Cargo.toml

@@ -48,6 +48,7 @@ rand = "0.8"
 axum-extra = { version = "0.9", features = ["multipart"] }
 tokio-util = { version = "0.7", features = ["io"] }
 zstd = "0.13"
+hex = "0.4"
 toml = "0.8"
 uuid = { version = "1", features = ["v4"] }
 dashmap = "6.1"