feat(backend): BACKEND_SLOT_PINNING — per-phase engine slot ownership

Each pipeline phase owns one llama-server slot (steward 0, orchestrator
1, synthesizer 2), carried as id_slot in extra_body through the same
mechanism tool_choice already uses, so the phase's stable prompt prefix
stays in that slot's KV cache and a turn re-prefills only its new
tokens. Off by default; a no-op on the Claude backend and ignored by
Ollama, so the flag is safe on any backend and the cutover itself stays
a pure env swap.

The merge helper preserves existing extra_body keys — mutation-checked
(dropping the merge fails exactly the test written for it).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/core/config.py

@@ -147,6 +147,10 @@ class Config(BaseSettings):
         default=None,
         description="OpenAI-compatible embeddings host; falls back to OLLAMA_HOST so gen and embed can live on different servers",
     )
+    BACKEND_SLOT_PINNING: bool = Field(
+        default=False,
+        description="Pin each pipeline phase to a llama-server slot (steward=0, orchestrator=1, synthesizer=2) so a phase's stable prompt prefix stays in that slot's KV cache; no-op on Claude, ignored by Ollama",
+    )
 
     # Redis Memory Database
     REDIS_MEMORY_DB: int = Field(default=1, description="Redis database number for memory cache")