Add WebDAV Versioning (Phase 1-5): version control with history tracking

Features:
- WebDavVersioning: Version control using HashMap storage
- VersionInfo/VersionHistory: Version metadata structures
- create_version/get_version/delete_version operations
- restore_version: Restore from previous version
- SHA-256 checksum calculation
- 11 unit tests for all operations

Files:
- markbase-core/src/webdav_version.rs (391 lines)
- markbase-core/src/lib.rs (add module)

Tests: 309 passed (+11)

markbase-core/src/lib.rs

@@ -23,6 +23,7 @@ pub mod ssh_server;
 pub mod sync;
 pub mod vfs;
 pub mod webdav;
+pub mod webdav_version;
 
 #[cfg(test)]
 mod security_audit;