feat(litellm): repoint fleet reranker alias to bge-reranker-v2-m3 (Brokkr R43)

The incumbent Qwen3-Reranker-0.6B was measured actively harming 80/90
fleet queries on main+knowledge_base (and inverting the bare-name region
behind Worldtree #389) — no-reranker beat it 89/90 vs 56/90. Brokkr's R43
bake-off selected BAAI/bge-reranker-v2-m3 (A3): 90/90 top-10, mean rank
0.19, multilingual (XLM-R), ~1.2 GB lighter than the incumbent.

Control arm (A2 = same Qwen weights, seq-cls head) scored identical to the
incumbent, proving the fault is a training prior, not the serving head —
which cancelled the expensive Qwen3-4B arm before it cost a GPU seat.

Cutover boundary 2026-08-06T17:37:48Z. The qwen3-reranker alias and the
:8002 backend are retained for one-edit rollback. Adds the process audit
trail at docs/pfi/reranker-selection-ledger.md.

stacks/litellm/conf/config.yaml

@@ -248,11 +248,13 @@ model_list:
       top_p: 0.9
     model_info:
       mode: chat
-  # reranker → generic capability name for rerank (currently qwen3-reranker).
+  # reranker → generic capability name for rerank. Repointed to bge-reranker-v2-m3
+  # 2026-08-06 (Brokkr R43: incumbent Qwen3-Reranker-0.6B harmed 80/90 fleet queries;
+  # boundary 17:37:48Z). Incumbent stays reachable via the qwen3-reranker alias for rollback.
   - model_name: reranker
     litellm_params:
-      model: hosted_vllm/Qwen/Qwen3-Reranker-0.6B
-      api_base: http://10.250.50.54:8002/v1
+      model: hosted_vllm/BAAI/bge-reranker-v2-m3
+      api_base: http://10.250.50.54:8013/v1
       api_key: os.environ/VLLM_API_KEY
     model_info:
       mode: rerank