[OSS26-Team] Bubble.List / Conversations 大列表虚拟滚动

## 任务：Bubble.List / Conversations 大列表虚拟滚动
- **标签**：`OSS26-Team`、`enhancement`
- **关联**：#1639、#1871
- **模块**：`packages/x/components/bubble/BubbleList.tsx`、`packages/x/components/conversations/`
- **建议组队人数**：2-3 人
- **基本要求**：熟悉虚拟列表（`rc-virtual-list` / `react-window`）、动态高度测量、滚动锚定。
- **目标产出**：
1. 两组件加可选虚拟滚动（不破坏现有 API）；
2. 兼容动态高度与「自动滚到底部」；
3. 1000+ 项 benchmark；
4. demo + 文档 + 单测。

Start by reading packages/x/components/bubble/BubbleList.tsx and the packages/x/components/conversations/ module, then review the related issues #1639 and #1871. Define an optional virtualization approach that preserves the existing API, supports dynamic heights and bottom anchoring, and validate it with a 1000+ item benchmark, demo, documentation, and unit tests.

Written by the indexing model from the issue text.