fix: metadata batch reading from correct directory

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.

src/storage/adapters/baseStorageAdapter.ts

@@ -38,6 +38,8 @@ export abstract class BaseStorageAdapter implements StorageAdapter {
 
   abstract getMetadata(id: string): Promise<any | null>
 
+  abstract getNounMetadata(id: string): Promise<any | null>
+
   abstract saveVerbMetadata(id: string, metadata: any): Promise<void>
 
   abstract getVerbMetadata(id: string): Promise<any | null>