Add SMB Share Snapshots (Phase 1-4): FSCTL_SRV_SNAPSHOT_* handlers

Features:
- SnapshotManager: Share snapshot management
- SnapshotEntry/SnapshotState: Snapshot metadata structures
- FSCTL_SRV_SNAPSHOT_CREATE/READ/WRITE/DELETE handlers
- GMT token format support (@GMT-YYYY.MM.DD-HH.MM.SS)
- 7 unit tests for all operations

Files:
- vendor/smb-server/src/snapshot.rs (245 lines)
- vendor/smb-server/src/handlers/ioctl.rs (+88 lines)
- vendor/smb-server/src/proto/messages/ioctl.rs (+8 lines enum)
- vendor/smb-server/src/server.rs (+2 lines)
- vendor/smb-server/src/ntstatus.rs (+1 line)
- vendor/smb-server/src/lib.rs (+1 line)

Tests: 7 passed (smb-server), 309 passed (markbase-core)

vendor/smb-server/src/lib.rs

@@ -31,6 +31,7 @@ mod oplock;
 mod path;
 mod proto;
 mod server;
+mod snapshot;
 mod utils;
 
 pub use backend::{BackendCapabilities, DirEntry, FileInfo, FileTimes, Handle, NullHandle, OpenIntent, OpenOptions, ShareBackend};