feat(#000057): arborist+qwen enablement — multi-engine JSON-schema + per-model extras pass-through

Two surgical fixes unblock 'arborist with synthesis LLM = Qwen-on-
llama.cpp' as a viable arm in the control sweep. Pre-existing
docstring said 'Arborist×Qwen needs proof-path guided_json+extra_body
surgery — coupled follow-up'; this is that follow-up.

Fix 1 — multi-engine structured-output extras

The runner / query JSON-mode paths previously sent only vLLM's
'guided_json' key for the claim_lattice schema. llama.cpp silently
drops it, leaving Qwen un-enforced (the parse-tolerant fallback did
all the work). Helper

    claim_lattice_structured_output_extras() in arborist/qa/verify.py

now returns a dict carrying the schema under all three engine
conventions:

  - guided_json     (vLLM grammar-constrained sampling)
  - json_schema     (llama.cpp native shorthand)
  - response_format (OpenAI-spec, honoured by llama.cpp and newer vLLM)

Each engine recognises its own key and silently drops the others.
Used at both inference call sites (runner.py:740, query.py:3324).
Hermes/vLLM path is unchanged — it picks up 'guided_json' and
ignores the other two.

Fix 2 — query() accepts user-supplied extra_body, merges with defaults

query() grew a keyword-only extra_body parameter (default None).
Per-model knobs (Qwen's {'chat_template_kwargs': {'enable_thinking':
False}} toggle, future template knobs) can flow from the caller to
the synthesis chat-completion call. Schema-enforcement extras are
added inside query() and merge under user keys — common case is
disjoint namespaces, but if a caller wants to override 'guided_json'
they can.

bench/control_sweep.py now passes MODELS[arborist_ref]['extra']
through to query() in the arborist branch, so --arborist-ref
qwen-nothink runs with reasoning disabled and --arborist-ref
qwen-think runs with reasoning enabled. Phase 1's arborist arm with
--arborist-ref=hermes is unaffected (MODELS['hermes']['extra'] is
None, merges to no-op).

Tests
  + 3 new in tests/test_verify_json.py covering helper default shape,
    alternate-schema reuse, and query()'s new extra_body parameter
  220 affected tests still green (verify / claim_lattice / judge /
    runner suite)
  pytest test_verify_json: 27/27

Next: small smoke run --arborist-ref qwen-nothink against 4-8 items
to confirm end-to-end before any full sweep. Phase 2 (qwen-think solo)
still running in background, unaffected — it doesn't touch the
arborist arm.

tests/test_verify_json.py

@@ -523,3 +523,64 @@ def test_verify_json_subject_tokens_absent_below_threshold_passes():
         f"expected STRICT (below threshold); got {v['audit_mode']} "
         f"with violations {[vio['kind'] for vio in v['violations']]}"
     )
+
+
+# --- multi-engine structured-output extras (claim_lattice JSON mode) -------
+
+def test_claim_lattice_structured_output_extras_default_shape():
+    """The helper returns extras under all three engine conventions so
+    the same call site enforces the JSON schema across vLLM, llama.cpp,
+    and OpenAI-spec endpoints. Engines silently drop unknown keys, so
+    sending all three is safe."""
+    from arborist.qa.verify import (
+        CLAIM_LATTICE_JSON_SCHEMA,
+        claim_lattice_structured_output_extras,
+    )
+
+    e = claim_lattice_structured_output_extras()
+    assert set(e.keys()) == {"guided_json", "json_schema", "response_format"}
+
+    # vLLM convention — the schema directly under `guided_json`.
+    assert e["guided_json"] is CLAIM_LATTICE_JSON_SCHEMA
+
+    # llama.cpp shorthand — the schema directly under `json_schema`.
+    assert e["json_schema"] is CLAIM_LATTICE_JSON_SCHEMA
+
+    # OpenAI-spec — wrapped under response_format with the type tag.
+    rf = e["response_format"]
+    assert rf["type"] == "json_schema"
+    inner = rf["json_schema"]
+    assert inner["name"] == "claim_lattice"
+    assert inner["schema"] is CLAIM_LATTICE_JSON_SCHEMA
+    assert inner["strict"] is True
+
+
+def test_claim_lattice_structured_output_extras_accepts_alternate_schema():
+    """The helper isn't claim-lattice-specific in mechanism — pass a
+    different schema and an alternate name to reuse the multi-engine
+    bridge for any structured-output feature."""
+    from arborist.qa.verify import claim_lattice_structured_output_extras
+
+    other = {"type": "object", "properties": {"x": {"type": "string"}},
+             "required": ["x"], "additionalProperties": False}
+    e = claim_lattice_structured_output_extras(other, name="my_thing")
+    assert e["guided_json"] is other
+    assert e["json_schema"] is other
+    assert e["response_format"]["json_schema"]["name"] == "my_thing"
+    assert e["response_format"]["json_schema"]["schema"] is other
+
+
+def test_query_accepts_extra_body_parameter():
+    """query() grew an extra_body keyword so per-model knobs (Qwen's
+    enable_thinking, etc.) can flow through to the synthesis call.
+    Schema-enforcement extras are added inside query() and merge with
+    the user-supplied dict (user keys override defaults on collision —
+    common case is disjoint namespaces)."""
+    import inspect
+    from arborist.qa.query import query
+
+    sig = inspect.signature(query)
+    assert "extra_body" in sig.parameters
+    p = sig.parameters["extra_body"]
+    # keyword-only with default None — accepts any caller, optional.
+    assert p.default is None