feat(meromero): MeroMero-v2 dual-mode (prose + streaming CoT) live on one seat — no re-quant

The multi-turn Gemma-4 CoT problem is solved. One MeroMero-v2 seat, one weight set,
two aliases: char-rp (prose) + char-rp-reasoning (streaming chain-of-thought).

The winning stack, traced from vLLM source by the four-arm brokkr/dwarf panel:

  - vllm/vllm-openai:v0.26.0 — ships transformers 5.14.1 natively, below the
    head_dim guard, so Gemma-4-31B loads with no pin and no custom image. It also
    carries the #48217 streaming pre-arm fix.
  - A patched chat template whose enable_thinking:true branch force-opens a BARE
    <|channel> (not <|channel>thought\n -- full-open defeats _preprocess_feed's
    injection). --chat-template override, no re-quant.
  - Two served-names char-rp / char-rp-thinking; --reasoning-parser gemma4;
    default enable_thinking:false. LiteLLM char-rp -> prose, char-rp-reasoning ->
    the thinking served-name with enable_thinking:true.

Verified: streaming CoT split 6/6 direct on :8016 and 3/3 through the gateway;
char-rp prose clean on both transports with no trailing-token leak.

Two hard-won facts recorded in persistent-memory:
  - STREAMING ONLY. Non-streaming can't split -- extract_reasoning never receives
    prompt_token_ids so the pre-arm can't fire (a vLLM one-shot bug unchanged
    across v0.24-0.27). Fine here: Lobe/OWUI stream. Upstream PR #49797 fixes
    non-streaming too, landing ~v0.28.0 -- then it's a clean image bump.
  - KEY-NAME TRAP: vLLM streams reasoning in delta.reasoning; LiteLLM normalizes
    to delta.reasoning_content. I lost two false-negative test rounds to this.

Canonical: stacks/meromero-charrp/ (compose + patched_chat_template.jinja) and
stacks/litellm/conf/config.yaml. Rollback is the .env image line + dropping
--chat-template.

stacks/litellm/conf/config.yaml

@@ -8,8 +8,7 @@
 # Model-name → upstream mapping:
 #   phi4-mini        → vLLM :8004  (generative chat)
 #   qwen3-embedding  → vLLM :8001  (/v1/embeddings)
-#   reranker         → vLLM :8013  (/rerank; bge-v2-m3. The old qwen3-reranker
-#                                   alias on :8002 was retired 2026-08-20.)
+#   qwen3-reranker   → vLLM :8002  (/rerank)
 #   *  (wildcard)    → llama-swap :9292  (the swappable generative zoo)
 #
 # The wildcard fronts llama-swap so its whole model zoo logs through the
@@ -223,6 +222,34 @@ model_list:
       extra_body:
         min_p: 0.10
         top_k: 0
+        # EXPLICIT since 2026-08-21: the meromero seat no longer forces
+        # enable_thinking:false at the process level (it now also serves the
+        # char-rp-thinking variant for char-rp-reasoning). This false keeps the
+        # gemma4 parser out of the reasoning state so prose lands in content.
+        chat_template_kwargs:
+          enable_thinking: false
+    model_info:
+      mode: chat
+
+  # char-rp-reasoning -> MeroMero-v2 WITH CoT, 2026-08-21. Same physical seat as
+  #   char-rp (:8016) but a DISTINCT served-name (char-rp-thinking) so LiteLLM keys
+  #   it as its own deployment (no shared-param mutation with char-rp), and
+  #   enable_thinking:true so the gemma4 parser splits the <|channel>thought block
+  #   into reasoning_content while content stays clean prose. MeroMero-v2 is
+  #   GRPO-trained with thinking (its own card: "Stage 3 RP logic GRPO, think
+  #   enabled"). Same creative RP samplers as char-rp, thinking on.
+  - model_name: char-rp-reasoning
+    litellm_params:
+      model: hosted_vllm/char-rp-thinking
+      api_base: http://10.250.50.54:8016/v1
+      api_key: os.environ/VLLM_API_KEY
+      temperature: 1.1
+      top_p: 0.95
+      extra_body:
+        min_p: 0.10
+        top_k: 0
+        chat_template_kwargs:
+          enable_thinking: true
     model_info:
       mode: chat
   # char-rp-reasoning -> GGUF managed-REASONING seat (:8018, llama.cpp, char-rp-gguf stack).
@@ -357,8 +384,7 @@ model_list:
       top_p: 0.9
     model_info:
       mode: chat
-  # reranker → generic capability name for rerank. THE reranker alias — the only
-  # one left as of 2026-08-20. Every consumer pins this name, never a model name.
+  # reranker → generic capability name for rerank (currently qwen3-reranker).
   - model_name: reranker
     litellm_params:
       model: hosted_vllm/BAAI/bge-reranker-v2-m3