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

tests/unit/test_direct/test_entity_centric_kg.py

@@ -547,21 +547,21 @@ class TestServiceHelperFunctions:
     """Test cases for helper functions in service.py"""
 
     def test_create_term_with_uri_otype(self):
-        """Test create_term creates IRI Term for otype='u'"""
+        """Test create_term creates IRI Term for term_type='u'"""
         from trustgraph.query.triples.cassandra.service import create_term
         from trustgraph.schema import IRI
 
-        term = create_term('http://example.org/Alice', otype='u')
+        term = create_term('http://example.org/Alice', term_type='u')
 
         assert term.type == IRI
         assert term.iri == 'http://example.org/Alice'
 
     def test_create_term_with_literal_otype(self):
-        """Test create_term creates LITERAL Term for otype='l'"""
+        """Test create_term creates LITERAL Term for term_type='l'"""
         from trustgraph.query.triples.cassandra.service import create_term
         from trustgraph.schema import LITERAL
 
-        term = create_term('Alice Smith', otype='l', dtype='xsd:string', lang='en')
+        term = create_term('Alice Smith', term_type='l', datatype='xsd:string', language='en')
 
         assert term.type == LITERAL
         assert term.value == 'Alice Smith'
@@ -569,7 +569,7 @@ class TestServiceHelperFunctions:
         assert term.language == 'en'
 
     def test_create_term_with_triple_otype(self):
-        """Test create_term creates TRIPLE Term for otype='t' with valid JSON"""
+        """Test create_term creates TRIPLE Term for term_type='t' with valid JSON"""
         from trustgraph.query.triples.cassandra.service import create_term
         from trustgraph.schema import TRIPLE, IRI
         import json
@@ -581,7 +581,7 @@ class TestServiceHelperFunctions:
             "o": {"type": "i", "iri": "http://example.org/Bob"},
         })
 
-        term = create_term(triple_json, otype='t')
+        term = create_term(triple_json, term_type='t')
 
         assert term.type == TRIPLE
         assert term.triple is not None