sequenceDiagram
  participant C as Client
  participant API as Save API
  participant DB as Database
  C->>API: PUT /saves/{slotId} (save, If-Match)
  API->>API: 校验Schema/引用/数值
  API->>DB: 写入(事务)
  DB-->>API: ok
  API-->>C: 200 (new etag)
