GraphRAG Query-Time Explainability (#677)

Implements full explainability pipeline for GraphRAG queries, enabling
traceability from answers back to source documents.

Renamed throughout for clarity:
- provenance_callback → explain_callback
- provenance_id → explain_id
- provenance_collection → explain_collection
- message_type "provenance" → "explain"
- Queue name "provenance" → "explainability"

GraphRAG queries now emit explainability events as they execute:
1. Session - query text and timestamp
2. Retrieval - edges retrieved from subgraph
3. Selection - selected edges with LLM reasoning (JSONL with id +
   reasoning)
4. Answer - reference to synthesized response

Events stream via explain_callback during query(), enabling
real-time UX.

- Answers stored in librarian service (not inline in graph - too large)
- Document ID as URN: urn:trustgraph:answer:{session_id}
- Graph stores tg:document reference (IRI) to librarian document
- Added librarian producer/consumer to graph-rag service

- get_labelgraph() now returns (labeled_edges, uri_map)
- uri_map maps edge_id(label_s, label_p, label_o) →
  (uri_s, uri_p, uri_o)
- Explainability data stores original URIs, not labels
- Enables tracing edges back to reifying statements via tg:reifies

- Added serialize_triple() to query service (matches storage format)
- get_term_value() now handles TRIPLE type terms
- Enables querying by quoted triple in object position:
  ?stmt tg:reifies <<s p o>>

- Displays real-time explainability events during query
- Resolves rdfs:label for edge components (s, p, o)
- Traces source chain via prov:wasDerivedFrom to root document
- Output: "Source: Chunk 1 → Page 2 → Document Title"
- Label caching to avoid repeated queries

GraphRagResponse:
- explain_id: str | None
- explain_collection: str | None
- message_type: str ("chunk" or "explain")
- end_of_session: bool

trustgraph-base/trustgraph/provenance/:
- namespaces.py - Added TG_DOCUMENT predicate
- triples.py - answer_triples() supports document_id reference
- uris.py - Added edge_selection_uri()

trustgraph-base/trustgraph/schema/services/retrieval.py:
- GraphRagResponse with explain_id, explain_collection, end_of_session

trustgraph-flow/trustgraph/retrieval/graph_rag/:
- graph_rag.py - URI preservation, streaming answer accumulation
- rag.py - Librarian integration, real-time explain emission

trustgraph-flow/trustgraph/query/triples/cassandra/service.py:
- Quoted triple serialization for query matching

trustgraph-cli/trustgraph/cli/invoke_graph_rag.py:
- Full explainability display with label resolution and source tracing

trustgraph-base/trustgraph/messaging/translators/retrieval.py

@@ -90,13 +90,31 @@ class GraphRagResponseTranslator(MessageTranslator):
     def from_pulsar(self, obj: GraphRagResponse) -> Dict[str, Any]:
         result = {}
 
-        # Include response content (even if empty string)
+        # Include message_type
+        message_type = getattr(obj, "message_type", "")
+        if message_type:
+            result["message_type"] = message_type
+
+        # Include response content for chunk messages
         if obj.response is not None:
             result["response"] = obj.response
 
-        # Include end_of_stream flag
+        # Include explain_id for explain messages
+        explain_id = getattr(obj, "explain_id", None)
+        if explain_id:
+            result["explain_id"] = explain_id
+
+        # Include explain_collection for explain messages
+        explain_collection = getattr(obj, "explain_collection", None)
+        if explain_collection:
+            result["explain_collection"] = explain_collection
+
+        # Include end_of_stream flag (LLM stream complete)
         result["end_of_stream"] = getattr(obj, "end_of_stream", False)
 
+        # Include end_of_session flag (entire session complete)
+        result["end_of_session"] = getattr(obj, "end_of_session", False)
+
         # Always include error if present
         if hasattr(obj, 'error') and obj.error and obj.error.message:
             result["error"] = {"message": obj.error.message, "type": obj.error.type}
@@ -105,5 +123,6 @@ class GraphRagResponseTranslator(MessageTranslator):
 
     def from_response_with_completion(self, obj: GraphRagResponse) -> Tuple[Dict[str, Any], bool]:
         """Returns (response_dict, is_final)"""
-        is_final = getattr(obj, 'end_of_stream', False)
+        # Session is complete when end_of_session is True
+        is_final = getattr(obj, 'end_of_session', False)
         return self.from_pulsar(obj), is_final