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>

tests/anthropic/test_model_selector.py

@@ -91,3 +91,32 @@ class TestGetModelInfo:
         info = model_selector.get_model_info()
         assert info["backend"] == "claude"
         assert info["model"] == config.ANTHROPIC_MODEL
+
+
+class TestWithSlotPinning:
+    def test_disabled_returns_settings_unchanged(self, local_first, monkeypatch):
+        monkeypatch.setattr(config, "BACKEND_SLOT_PINNING", False)
+        base = model_selector.get_tool_choice_settings()
+        assert model_selector.with_slot_pinning(base, slot=1) is base
+        assert model_selector.with_slot_pinning(None, slot=2) is None
+
+    def test_enabled_merges_slot_with_existing_extra_body(self, local_first, monkeypatch):
+        monkeypatch.setattr(config, "BACKEND_SLOT_PINNING", True)
+        settings = model_selector.with_slot_pinning(
+            model_selector.get_tool_choice_settings(), slot=1
+        )
+        extra_body = settings["extra_body"]
+        assert extra_body["id_slot"] == 1
+        # tool_choice from the base settings survives the merge
+        assert extra_body["tool_choice"] == "required"
+
+    def test_enabled_pins_bare_settings(self, local_first, monkeypatch):
+        monkeypatch.setattr(config, "BACKEND_SLOT_PINNING", True)
+        settings = model_selector.with_slot_pinning(None, slot=2)
+        assert settings["extra_body"] == {"id_slot": 2}
+
+    def test_claude_backend_never_pinned(self, local_first, monkeypatch):
+        monkeypatch.setattr(config, "BACKEND_SLOT_PINNING", True)
+        monkeypatch.setattr(config, "PREFER_CLOUD_BACKEND", True)
+        base = model_selector.get_tool_choice_settings()
+        assert model_selector.with_slot_pinning(base, slot=1) is base