feat: 多专业小模型+路由模型系统 MVP（mock 全链路 + FastAPI 网关 + 论文调研）

router_system/config.py

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+"""配置加载：优先 YAML（若安装了 pyyaml），否则回退 JSON。
+
+设计原则：router_system 核心零依赖，因此 pyyaml 是"可选"的。
+默认 config/config.yaml 存在；若 pyyaml 不可用，可提供同名 .json。
+"""
+from __future__ import annotations
+
+import json
+import os
+from pathlib import Path
+from typing import Any, Dict, Optional
+
+DEFAULT_CONFIG_PATH = Path(__file__).resolve().parent.parent / "config" / "config.yaml"
+
+_DEFAULTS: Dict[str, Any] = {
+    "system": {"name": "multi-expert-router", "version": "0.1.0"},
+    "router": {
+        "low_confidence_threshold": 0.60,   # 分类置信度低于此值 -> 直接走大模型
+        "judge_fallback_threshold": 0.70,   # Judge 质量分低于此值 -> 升级大模型
+        "default_temperature": 0.2,
+    },
+    "classifier": {"type": "rule", "model": "Qwen/Qwen3-0.6B", "confidence_floor": 0.55},
+    "domains": ["code", "math", "legal", "medical", "general"],
+    "experts": {
+        "code": {"type": "mock", "model": "Qwen/Qwen2.5-Coder-7B-Instruct"},
+        "math": {"type": "mock", "model": "Qwen/Qwen3-4B-Instruct"},
+        "legal": {"type": "mock", "model": "Qwen/Qwen3-4B-Instruct"},
+        "medical": {"type": "mock", "model": "Qwen/Qwen3-4B-Instruct"},
+        "general": {"type": "mock", "model": "Qwen/Qwen3-1.7B-Instruct"},
+    },
+    "fallback": {
+        "type": "mock",
+        "model": "deepseek-chat",
+        "base_url": "https://api.deepseek.com/v1",
+        "api_key_env": "DEEPSEEK_API_KEY",
+    },
+    "judge": {"type": "rule", "model": "Qwen/Qwen3-1.7B-Instruct"},
+    "cache": {
+        "enabled": True,
+        "semantic_enabled": True,
+        "similarity_threshold": 0.88,
+        "promote_frequency": 5,
+    },
+}
+
+
+def load_defaults() -> Dict[str, Any]:
+    return _DEFAULTS
+
+
+def _try_load_yaml(path: Path) -> Optional[Dict[str, Any]]:
+    try:
+        import yaml  # type: ignore
+    except ImportError:
+        return None
+    try:
+        with open(path, "r", encoding="utf-8") as f:
+            data = yaml.safe_load(f)
+        return data if isinstance(data, dict) else None
+    except Exception:
+        return None
+
+
+def _try_load_json(path: Path) -> Optional[Dict[str, Any]]:
+    json_path = path.with_suffix(".json")
+    if not json_path.exists():
+        return None
+    try:
+        with open(json_path, "r", encoding="utf-8") as f:
+            data = json.load(f)
+        return data if isinstance(data, dict) else None
+    except Exception:
+        return None
+
+
+def _merge_defaults(data: Dict[str, Any]) -> Dict[str, Any]:
+    """将用户配置与内置默认配置做一层合并（用户优先）。"""
+    merged = dict(_DEFAULTS)
+    for k, v in data.items():
+        if isinstance(v, dict) and isinstance(merged.get(k), dict):
+            merged[k] = {**merged[k], **v}
+        else:
+            merged[k] = v
+    return merged
+
+
+def load_config(path: Optional[Path | str] = None) -> Dict[str, Any]:
+    """加载配置，返回 dict。文件不存在或解析失败时返回内置默认配置。"""
+    cfg_path = Path(path) if path else DEFAULT_CONFIG_PATH
+    if cfg_path.exists():
+        data = _try_load_yaml(cfg_path) or _try_load_json(cfg_path)
+        if data is not None:
+            return _merge_defaults(data)
+    return dict(_DEFAULTS)
+
+
+def get_api_key(cfg: Dict[str, Any]) -> Optional[str]:
+    """从环境变量读取 API Key（用于 api 类型后端）。"""
+    env_name = cfg.get("api_key_env") or "API_KEY"
+    return os.environ.get(env_name) or None␍