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/defect_classes.py

@@ -0,0 +1,33 @@
+from typing import Dict
+
+DEFECT_CLASSES: Dict[int, dict] = {
+    # HRIPCB 数据集原始 6 类
+    0: {"name": "missing_hole",    "zh": "缺孔",    "severity": "fatal"},
+    1: {"name": "mouse_bite",      "zh": "鼠咬",    "severity": "major"},
+    2: {"name": "open_circuit",    "zh": "断路",    "severity": "fatal"},
+    3: {"name": "short_circuit",   "zh": "短路",    "severity": "fatal"},
+    4: {"name": "spur",            "zh": "毛刺",    "severity": "minor"},
+    5: {"name": "spurious_copper", "zh": "余铜",    "severity": "major"},
+    # 扩展类（Phase 2 标注后加入训练）
+    6: {"name": "oxidation",       "zh": "氧化",    "severity": "minor"},
+    7: {"name": "solder_ball",     "zh": "锡珠",    "severity": "rework"},
+    8: {"name": "scratch",         "zh": "划痕",    "severity": "minor"},
+    9: {"name": "label_error",     "zh": "标签错贴","severity": "rework"},
+}
+
+# 严重程度从低到高，用于比较 max_severity
+SEVERITY_ORDER = ["none", "rework", "minor", "major", "fatal"]
+
+
+def get_class_info(class_id: int) -> dict:
+    return DEFECT_CLASSES.get(class_id, {
+        "name": f"unknown_{class_id}",
+        "zh": f"未知_{class_id}",
+        "severity": "minor",
+    })
+
+
+def max_severity(severities) -> str:
+    if not severities:
+        return "none"
+    return max(severities, key=lambda s: SEVERITY_ORDER.index(s) if s in SEVERITY_ORDER else 0)