VFS files store content in BlobStorage, but versioning was capturing
stale embedding text from entity.data instead of actual file content.
Changes:
- VersionManager.save() now reads fresh content via vfs.readFile()
- VersionManager.restore() writes content back via vfs.writeFile()
- Text files stored as UTF-8, binary as base64 with encoding flag
- Added comprehensive VFS versioning test suite (10 tests)
- Updated API docs with VFS file versioning example
Fixes: Workshop team bug report where all VFS file versions had
identical data despite different metadata.size values.
