feat(creator-studio): 前台 Gutenberg 创作工作台与受保护付费附件

前台投稿页改为沉浸式创作工作台：Gutenberg 负责区块解析与序列化，
React + TypeScript 负责工作台外壳与发布抽屉，Jio 服务端继续作为权限、
审核与付费的唯一权威。

- 新增 assets/creator-studio 独立构建（wp-scripts，锁定 WP 6.4 时代包），
  仅在投稿模板加载，构建缺失或游客访问时回退原有 TinyMCE 表单
- 自动保存（4s 防抖 + rev 冲突 409）、本地快照恢复、发布配置抽屉、
  移动端全屏单列与安全区适配
- 图片插入走现有 front-submit 上传管线，保留能力校验与待认领生命周期
- 已发布文章的改稿写入私有版本，审核通过前线上内容/链接/付费条款不变；
  原生后台「改稿审核」一次性应用或退回，并校验源文章并发指纹
- 私有附件新表 jio_private_asset + jio_dl_token.asset_id，schema 升到 3.4.0；
  文件存放 ABSPATH 之外，未配置私有存储时 fail closed
- 下载令牌绑定文章/附件/用户/IP/UA/次数，每次交付复核 Visibility 的
  订单、VIP 与配额判定；中断传输归还次数
- 付费字段改为写入前校验，非法价格不再留下半保存文章
- 新增 creator-studio 契约测试；更新受影响的 schema/下载/投稿契约

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

assets/creator-studio/build/index.css

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