Fixed bug where getMetadataBatch() was reading from wrong directory:
- FileSystemStorage: Changed to use getNounMetadata() instead of getMetadata()
- OPFSStorage: Changed to use getNounMetadata() instead of getMetadata()
- MetadataIndex fallback: Fixed to use getNounMetadata()
- Added getNounMetadata() to StorageAdapter interface
This resolves 0% success rate during metadata index rebuild.
Also added comprehensive API documentation for return values and data field behavior.
