新增 external v1 游戏场景生成路由并迁移 AGC 美术包背景阶段

- api-server 新增 POST /api/external/v1/editor/scenes/generations，复用 editor:image-generate scope、幂等键和站内场景生图队列
- editor_project.rs 抽取站内与外部共用的场景生图 payload 构造函数，站内 handler 改为调用共享函数
- 同步更新 external v1 OpenAPI 契约与 MCP 派生排除标记，登记 docs/README.md 索引
- AGC 客户端美术包背景阶段改走新场景路由，stylePreset 固定 custom + customStyle 保留现有风格文案
- external_generation_state 快照与 recovery_scan 账本白名单支持新场景路由
- direct_runtime 背景资源识别同时兼容新场景路由与旧通用路由，新增旧路由回放测试
- 更新 genarrative-external-editor-api skill 参考文档
- 新增主规范、里程碑规范与实施计划三份 SDD 文档

docs/openapi/genarrative-external-v1.openapi.json

@@ -1052,7 +1052,60 @@
             "$ref": "#/components/responses/UpstreamError"
           }
         },
-        "description": "支持普通生图、规范图、角色图、快速编辑参考图、UI 设计图和宣发素材生成。kind 可取 spec、character、quick-edit、ui-design、publication-material。"
+        "description": "支持普通生图、规范图、角色图、快速编辑参考图、UI 设计图和宣发素材生成。kind 可取 spec、character、quick-edit、ui-design、publication-material。游戏场景不接受本接口的 kind/assetKind = scene，必须使用 /api/external/v1/editor/scenes/generations 提交结构化场景意图。"
+      }
+    },
+    "/api/external/v1/editor/scenes/generations": {
+      "post": {
+        "x-mcp-excluded": true,
+        "tags": ["Editor Images"],
+        "operationId": "generateExternalEditorScene",
+        "summary": "生成编辑器游戏场景（结构化场景意图）",
+        "description": "只接受结构化场景意图：sceneContent + stylePreset（custom 时必须提供 customStyle），完整 Provider Prompt 由服务端组装，不接受调用方提交的完整 prompt。入队后按 kind/assetKind = scene 持久化，产物保存 scene.generate V2 配方；队列、计费、资源入库与画布写回与站内场景路由一致。",
+        "security": [
+          {
+            "ExternalApiKey": []
+          }
+        ],
+        "parameters": [
+          {
+            "$ref": "#/components/parameters/IdempotencyKey"
+          }
+        ],
+        "requestBody": {
+          "required": true,
+          "content": {
+            "application/json": {
+              "schema": {
+                "$ref": "#/components/schemas/EditorSceneGenerationRequest"
+              }
+            }
+          }
+        },
+        "responses": {
+          "202": {
+            "description": "生成任务已持久化入队",
+            "content": {
+              "application/json": {
+                "schema": {
+                  "$ref": "#/components/schemas/ExternalEditorGenerationSubmissionResponse"
+                }
+              }
+            }
+          },
+          "400": {
+            "$ref": "#/components/responses/BadRequest"
+          },
+          "401": {
+            "$ref": "#/components/responses/Unauthorized"
+          },
+          "403": {
+            "$ref": "#/components/responses/Forbidden"
+          },
+          "502": {
+            "$ref": "#/components/responses/UpstreamError"
+          }
+        }
       }
     },
     "/api/external/v1/editor/images/edits": {
@@ -2965,6 +3018,68 @@
           }
         }
       },
+      "EditorSceneGenerationRequest": {
+        "type": "object",
+        "required": ["sceneContent", "stylePreset"],
+        "properties": {
+          "sceneContent": {
+            "type": "string",
+            "minLength": 1,
+            "description": "画面内容（结构化场景意图主体）。纯空白在入队前返回 400。"
+          },
+          "stylePreset": {
+            "type": "string",
+            "enum": ["anime", "watercolor", "flat", "stop-motion", "custom"],
+            "description": "视觉风格预设。custom 时必须同时提供非空 customStyle，否则返回 400。"
+          },
+          "customStyle": {
+            "type": ["string", "null"],
+            "description": "自定义画风描述，仅 stylePreset = custom 时使用。"
+          },
+          "model": {
+            "type": ["string", "null"],
+            "description": "图片模型，省略时使用服务端默认场景模型。"
+          },
+          "aspectRatio": {
+            "type": ["string", "null"],
+            "default": "16:9"
+          },
+          "imageSize": {
+            "type": ["string", "null"],
+            "default": "1K"
+          },
+          "referenceImageSrcs": {
+            "type": "array",
+            "items": {
+              "type": "string"
+            },
+            "description": "可选参考图，沿用普通图片生成的参考图口径。"
+          },
+          "projectId": {
+            "type": ["string", "null"]
+          },
+          "generationInputs": {
+            "$ref": "#/components/schemas/JsonValue"
+          },
+          "assetFolderId": {
+            "type": ["string", "null"]
+          },
+          "assetLabel": {
+            "type": ["string", "null"],
+            "description": "省略或纯空白时统一使用「游戏场景」。"
+          },
+          "canvasCompletion": {
+            "anyOf": [
+              {
+                "$ref": "#/components/schemas/EditorCanvasGenerationCompletion"
+              },
+              {
+                "type": "null"
+              }
+            ]
+          }
+        }
+      },
       "EditorImageGenerationRequest": {
         "type": "object",
         "required": ["prompt"],