The id field in pipeline Metadata was being overwritten at each processing (#686)

The id field in pipeline Metadata was being overwritten at each processing
stage (document → page → chunk), causing knowledge storage to create
separate cores per chunk instead of grouping by document.

Add a root field that:
- Is set by librarian to the original document ID
- Is copied unchanged through PDF decoder, chunkers, and extractors
- Is used by knowledge storage for document_id grouping (with fallback to id)

Changes:
- Add root field to Metadata schema with empty string default
- Set root=document.id in librarian when initiating document processing
- Copy root through PDF decoder, recursive chunker, and all extractors
- Update knowledge storage to use root (or id as fallback) for grouping
- Add root handling to translators and gateway serialization
- Update test mock Metadata class to include root parameter

trustgraph-base/trustgraph/schema/core/metadata.py

@@ -5,6 +5,9 @@ class Metadata:
     # Source identifier
     id: str = ""
 
+    # Root document identifier (set by librarian, preserved through pipeline)
+    root: str = ""
+
     # Collection management
     user: str = ""
     collection: str = ""