"Feature Request: Infinite Scroll Up Support for bubble.list"

### 需求动机

### Description
It would be a great enhancement to add **infinite scroll upwards** support for `bubble.list`. This is especially useful for chat applications or similar use cases where users scroll up to load older content.

Currently, `bubble.list` supports regular infinite scrolling downwards, but there is no built-in support for upwards scrolling.

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### Use Case
For chat-based applications or timelines, when users reach the top of the scroll container, they expect older data to load seamlessly. Implementing infinite scroll upwards will greatly improve user experience.

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### Expected Behavior
1. When the user scrolls to the **top** of the `bubble.list`:
- A callback function like `onScrollUp` is triggered.
- New data is fetched and prepended to the list without shifting the scroll position abruptly.

2. Maintain the current scroll position to avoid user confusion.

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### 提议的 API 是什么样的？

### Proposed API
Add a prop like:
```tsx

Start by locating the bubble.list implementation and its existing downward infinite-scroll entry point. Review how loading props and scroll state are handled; done means an upward-loading API can fetch older content, prepend it, show loading state, and preserve the user's visible position.

Written by the indexing model from the issue text.