接口5：改为多选发型 + 每发型返回 middle/high/low 三档叠图（去掉 hairline_level 入参）

- 去掉 hairline_level 入参；middle/high/low 三档都返回
- 入参改为同接口2：gender + hair_style(逗号分隔多选，必填)，缺失/越界返回 1007
- 每个选中发型返回 image_{middle,high,low}_base64 三档叠图 + grown_image_base64 生发图，
  按发型分组；order = 发型序号，含 hairline_type
- 生发黑模板仍固定 middle(hairline_texture_black/)，每发型 1 张生发图
- best_hairline_center_point 取首个选中发型的 middle 档
- 同步更新测试页(三档并排展示)、integration.html、stub_worker、接口文档/实现说明、test_api

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

gateway/stub_worker.py

@@ -223,15 +223,24 @@ def create_app(port: int, password: str) -> FastAPI:
         if x_internal_token != password:
             return JSONResponse(status_code=401, content={"detail": "unauthorized"})
         delay = float(request.query_params.get("delay", "1"))
+        _png = f"data:image/png;base64,{TINY_PNG_BASE64}"
         data = {
             "hairline_images": [
                 {
-                    "image_base64": f"data:image/png;base64,{TINY_PNG_BASE64}",
+                    "hairline_type": "ellipse",
+                    "image_middle_base64": _png,
+                    "image_high_base64": _png,
+                    "image_low_base64": _png,
+                    "grown_image_base64": _png,
                     "order": 1,
                 },
                 {
-                    "image_base64": f"data:image/png;base64,{TINY_PNG_BASE64}",
-                    "order": 2,
+                    "hairline_type": "heart",
+                    "image_middle_base64": _png,
+                    "image_high_base64": _png,
+                    "image_low_base64": _png,
+                    "grown_image_base64": None,
+                    "order": 3,
                 },
             ],
             "best_hairline_center_point": {"x": 540, "y": 430},