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_service.py

@@ -66,6 +66,7 @@ class TestDocumentRagService:
             workspace=ANY,            # Workspace comes from flow.workspace (mock)
             collection="test_coll_1", # Must be from message, not hardcoded default
             doc_limit=5,
+            fetch_limit=0,            # Unset -> core derives the candidate pool
             explain_callback=ANY,     # Explainability callback is always passed
             save_answer_callback=ANY, # Librarian save callback is always passed
         )