feat(sense): T-G0 语义分析器骨架（包结构/DDL/灰度门控挂载）

- gateway/sense/：config（SenseConfig：mode 灰度三态/特征门/conformal/consumers
  档位映射，mode 非法回落 collect）、errors（EmbedderDown/ArtifactMissing，D-G4 降级）、
  store（tier_observations + sense_artifacts DDL，WAL；观察写入/标签回填/
  labeled 查询/180d 清理/工件登记与 active 切换）、routes（/sense/health 透出灰度状态）
- settings DEFAULTS 增 sense 段（enabled 默认 False，mode 默认 collect——D-G7）
- api.py include_router 门控（装配失败不拖垮主应用）
- 测试 +5：门控 404/DDL 幂等/配置缺省与 mode 回落/独立挂载 health/
  观察写入-回填-计数-清理链路，全量 375 passed

gateway/settings.py

@@ -68,6 +68,21 @@ DEFAULTS: Dict[str, Any] = {
         "semcache": {"enabled": True, "sim_threshold": 0.92,
                      "max_entries": 300000, "promote_frequency": 5},
     },
+    # 语义分析器与三级分级（T-G0；结构见《实施方案_语义分析器与三级分级.md》§5）
+    "sense": {
+        "enabled": False,                    # D-G7 总开关：默认关
+        "mode": "collect",                   # collect | shadow | live（collect 攒满标签前禁 live）
+        "db_path": "data/sense.sqlite3",
+        "admin_key": "",
+        "embedder": {"base_url": "http://127.0.0.1:8902/v1",
+                     "model": "bge-m3-Q4_K_M", "timeout_s": 5, "dim": 1024},
+        "features": {"t1_max_tokens": 512, "t1_max_turns": 2,
+                     "intent_blacklist": ["重构", "脚手架", "迁移", "实现", "多文件", "项目"],
+                     "code_t1_kinds": ["解释", "补全"]},
+        "policy": {"alpha": 0.05, "min_labels": 500,
+                   "t2_prefer_local_when_idle": True},
+        "consumers": {"proxy": {"t1": "local-small", "t2": "budget", "t3": "premium"}},
+    },
 }