[WIP] Add field groups metadata layer MVP

[Claude]

Thanks for asking me to work on this. I will get started on it and keep this PR's description up to date as I form a plan and make progress.

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This section details on the original issue you should resolve

<issue_title>字段分组（fieldGroups）协议简化版：元数据层MVP</issue_title>
<issue_description>### 需求背景

• 目前已有对象设计器/协议规划，但字段分组相关协议设计过于复杂，AI生成/多包扩展等情况下难以维护、易错
• 需先落地MVP：首步只解决字段与分组的对应关系、分组元信息的有序持久化，顺序取自声明顺序

具体开发要求

1. ObjectSchema 上新增可选 fieldGroups: ObjectFieldGroup[]，定义分组信息（key/label/icon/description/defaultExpanded/visibleOn），数组顺序即分组顺序
2. FieldSchema 上沿用 group: string，表示字段所属分组。扩展包/界面新增字段/运行时���码一致用此分组归属
3. 分组到字段的映射由元数据注册顺序自动得出。组内显示顺序等于 ObjectSchema.fields 的遍历顺序
4. 变更操作迁移设计只需支持新增/重命名/删除/调整分组、字段加分组，组内顺序变更等功能待后续
5. 更新相关测试、文档和 CHANGELOG</issue_description>

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Project	Deployment	Actions	Updated (UTC)
objectstack-demo	Ready	Preview, Comment	Apr 20, 2026 2:03pm
spec	Ready	Preview, Comment	Apr 20, 2026 2:03pm