feat: QA 护栏 + fallback 路由（T15 架构审查整改收官，OV6）

- src/genesis/qa/guardrails.py: resolve_qa_model() 强制 QA 走 fallback 模型（防
  DeepSeek 校验 DeepSeek 自校验盲区）；QALoopController(max_rounds=3) 约束
  「QA→Writer修正→重校验」循环边界防无限
- 集成测试验证 QA 调用实际落到 fallback 而非 primary（FakeLLMClient 记录 model）
- design.md §7.4 补 QA 护栏说明（循环边界 + 独立校验模型）
- TDD：257 passed / 100.00% 覆盖，17 项架构审查整改全部完成

tests/test_qa_guardrails.py

@@ -0,0 +1,114 @@
+from __future__ import annotations
+
+import types as t
+
+import pytest
+
+from genesis.inference.engine import InferenceEngine
+from genesis.inference.token import approximate_token_count
+from genesis.qa.guardrails import (
+    DEFAULT_MAX_QA_ROUNDS,
+    QALoopController,
+    resolve_qa_model,
+)
+from tests.inference_helpers import FakeLLMClient
+
+
+class QAModels:
+    """模拟 config.InferenceModels：primary=deepseek / fallback=qwen。"""
+
+    def __init__(self):
+        self.primary = t.SimpleNamespace(name="deepseek-chat", provider="deepseek")
+        self.fallback = t.SimpleNamespace(name="qwen-max", provider="qwen")
+
+
+class QAModelsNoFallback:
+    """无 fallback 配置（覆盖 None 分支）。"""
+
+    def __init__(self):
+        self.primary = t.SimpleNamespace(name="deepseek-chat")
+        self.fallback = None
+
+
+class QAModelsFallbackEmptyName:
+    """fallback 存在但 name 为空字符串（覆盖 name 假值分支）。"""
+
+    def __init__(self):
+        self.primary = t.SimpleNamespace(name="deepseek-chat")
+        self.fallback = t.SimpleNamespace(name="")
+
+
+# ---------- resolve_qa_model（OV6：强制 fallback，防自校验盲区） ----------
+
+def test_resolve_qa_model_returns_fallback_name():
+    assert resolve_qa_model(QAModels()) == "qwen-max"
+
+
+def test_resolve_qa_model_never_returns_primary():
+    """关键护栏：QA 不得返回主模型名（否则 DeepSeek 校验 DeepSeek 盲区）。"""
+    models = QAModels()
+    assert resolve_qa_model(models) != getattr(models.primary, "name", None)
+
+
+def test_resolve_qa_model_no_fallback_returns_none():
+    """无 fallback 时返回 None，迫使调用方显式指定独立校验模型。"""
+    assert resolve_qa_model(QAModelsNoFallback()) is None
+
+
+def test_resolve_qa_model_fallback_empty_name_returns_none():
+    """fallback 存在但 name 为空时返回 None（不静默回退到 primary）。"""
+    assert resolve_qa_model(QAModelsFallbackEmptyName()) is None
+
+
+def test_resolve_qa_model_none_models_returns_none():
+    assert resolve_qa_model(None) is None
+
+
+# ---------- QALoopController（OV6：循环边界防无限） ----------
+
+def test_loop_default_max_rounds():
+    ctrl = QALoopController()
+    assert ctrl.max_rounds == DEFAULT_MAX_QA_ROUNDS
+
+
+def test_loop_stops_at_max_rounds():
+    ctrl = QALoopController(max_rounds=3)
+    assert ctrl.can_continue()
+    ctrl.advance()
+    ctrl.advance()
+    ctrl.advance()
+    assert ctrl.round == 3
+    assert ctrl.is_exhausted()
+    assert not ctrl.can_continue()
+
+
+def test_loop_exhausted_cannot_continue():
+    ctrl = QALoopController(max_rounds=1)
+    ctrl.advance()
+    assert ctrl.is_exhausted()
+    assert not ctrl.can_continue()
+
+
+# ---------- 集成：QA 调用强制走 fallback 模型 ----------
+
+@pytest.mark.anyio
+async def test_qa_call_uses_fallback_not_primary():
+    client = FakeLLMClient([("ok", "QA 通过")])
+    models = QAModels()
+    eng = InferenceEngine(
+        client=client,
+        models=models,
+        estimator=approximate_token_count,
+    )
+    qa_model = resolve_qa_model(models)
+    assert qa_model == "qwen-max"
+    await eng.chat(
+        session_id="qa1",
+        prompt="请校验以下章节",
+        variables={},
+        model=qa_model,
+    )
+    called_models = [c["model"] for c in client.calls]
+    # 实际 LLM 调用必须用 fallback，绝不可落到 primary
+    assert called_models == ["qwen-max"]
+    assert "deepseek-chat" not in called_models