feat(vision): 实现 mingxi-vision 推理服务核心功能

- engine/defect_classes: HRIPCB 6类缺陷定义 + 4类扩展 + severity排序
- engine/base: DefectBox / DetectResult 数据类 + BaseInferenceEngine 抽象基类
- engine/ultralytics_adapter: YOLOv8 .pt 推理适配器 + warmup
- engine/annotator: 按 severity 着色的标注图生成 + base64 编码
- engine/loader: 单例管理 + asyncio.Lock 推理串行化 + 热重载支持
- config: Pydantic v1 BaseSettings，读取 .env
- schema: DetectResponse / HealthResponse / ReloadRequest 等响应模型
- api/detect: POST /api/detect，内存读图，推理锁保护
- api/health: GET /api/health
- api/model: POST /api/model/reload 热重载接口
- main: FastAPI lifespan 启动加载模型

测试通过：/api/health 200，/api/detect 空图返回 pass，标注图 base64 正常

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

mingxi-vision/engine/base.py

@@ -0,0 +1,59 @@
+from abc import ABC, abstractmethod
+from dataclasses import dataclass
+from typing import List
+
+import numpy as np
+
+from .defect_classes import max_severity
+
+
+@dataclass
+class DefectBox:
+    class_id: int
+    class_name: str
+    class_name_zh: str
+    confidence: float
+    severity: str          # fatal / major / minor / rework / none
+    box_xyxy: List[float]
+
+
+@dataclass
+class DetectResult:
+    defects: List[DefectBox]
+    duration_ms: float
+    image_width: int
+    image_height: int
+    model_version: str
+
+    @property
+    def defect_count(self) -> int:
+        return len(self.defects)
+
+    @property
+    def max_severity(self) -> str:
+        return max_severity([d.severity for d in self.defects]) if self.defects else "none"
+
+    @property
+    def avg_confidence(self) -> float:
+        if not self.defects:
+            return 0.0
+        return round(sum(d.confidence for d in self.defects) / len(self.defects), 4)
+
+    @property
+    def verdict(self) -> str:
+        return "fail" if self.defects else "pass"
+
+
+class BaseInferenceEngine(ABC):
+    @abstractmethod
+    def detect(self, image: np.ndarray, conf: float = 0.45) -> DetectResult:
+        ...
+
+    @abstractmethod
+    def warmup(self) -> None:
+        ...
+
+    @property
+    @abstractmethod
+    def model_version(self) -> str:
+        ...