feat: add cross-encoder reranking to Document-RAG with two-limit control (#878) (#1011)

Wire the FlashRank reranker subsystem from #1005 into Document-RAG: after
vector retrieval, over-fetch a wider candidate pool, rerank with the
cross-encoder, and keep the top doc_limit chunks for synthesis.

Per maintainer review, the fetch and select sizes are two caller-controlled
limits rather than one internal heuristic:

- doc_limit:   chunks selected into the synthesis prompt (unchanged meaning).
- fetch_limit: candidate pool pulled from the vector store before reranking.
  0 = derive (OVERFETCH_FACTOR x doc_limit); values below doc_limit are
  raised to it. Lets the caller control how hard the reranker has to work.

Details:
- schema: DocumentRagQuery.fetch_limit (additive, backward compatible).
- document_rag.py / rag.py: fetch_limit resolved in the processor (mirrors
  doc_limit); the core applies the heuristic default and derives synthesis
  provenance from the chunk-selection focus when reranking ran.
- provenance: tg:ChunkSelection focus stage (mirrors tg:EdgeSelection).
- request translator + client SDKs + CLI: fetch-limit / --fetch-limit,
  threaded exactly like doc_limit and the GraphRAG limits.
- tests: no-op identity, over-fetch/narrow, explicit fetch_limit, heuristic
  default, floor-at-doc_limit, provenance lineage, cross-repo topic wiring.

Reranking is skipped byte-identically when no reranker role is wired.
Requires the companion trustgraph-templates change wiring the reranker
topics into the document-rag flow (mirrors #279 for GraphRAG).

tests/unit/test_retrieval/test_document_rag.py

@@ -101,27 +101,27 @@ class TestQuery:
         assert query.rag == mock_rag
         assert query.collection == "test_collection"
         assert query.verbose is False
-        assert query.doc_limit == 20  # Default value
+        assert query.fetch_limit == 20  # Default value
 
-    def test_query_initialization_with_custom_doc_limit(self):
-        """Test Query initialization with custom doc_limit"""
+    def test_query_initialization_with_custom_fetch_limit(self):
+        """Test Query initialization with custom fetch_limit"""
         # Create mock DocumentRag
         mock_rag = MagicMock()
 
-        # Initialize Query with custom doc_limit
+        # Initialize Query with custom fetch_limit
         query = Query(
             rag=mock_rag,
             workspace="test_workspace",
             collection="custom_collection",
             verbose=True,
-            doc_limit=50
+            fetch_limit=50
         )
 
         # Verify initialization
         assert query.rag == mock_rag
         assert query.collection == "custom_collection"
         assert query.verbose is True
-        assert query.doc_limit == 50
+        assert query.fetch_limit == 50
 
     @pytest.mark.asyncio
     async def test_extract_concepts(self):
@@ -224,7 +224,7 @@ class TestQuery:
             workspace="test_workspace",
             collection="test_collection",
             verbose=False,
-            doc_limit=15
+            fetch_limit=15
         )
 
         # Call get_docs with concepts list
@@ -377,7 +377,7 @@ class TestQuery:
             workspace="test_workspace",
             collection="test_collection",
             verbose=True,
-            doc_limit=5
+            fetch_limit=5
         )
 
         # Call get_docs with concepts
@@ -615,7 +615,7 @@ class TestQuery:
             workspace="test_workspace",
             collection="test_collection",
             verbose=False,
-            doc_limit=10
+            fetch_limit=10
         )
 
         docs, chunk_ids = await query.get_docs(["concept A", "concept B"])