refactor(appbar): improve MD3 variants and action composition

Refactor AppBar to improve Material Design 3 compliance, remove problematic children patterns, improve performance characteristics, and modernize the public API.

**Problems**

- Current API relies heavily on children composition in ways that are difficult to optimize
- Internal layout logic is tightly coupled to arbitrary child structures
- MD3 behaviors and visuals are not fully aligned with current Material guidelines
- Rendering performance can degrade in large lists or frequently updated screens
- Type safety and discoverability of the API can be improved
- The component has inconsistent behavior between platforms and edge cases

**Focus Areas**

- Improve support for MD3 top app bar variants
- Simplify action rendering
- Improve scroll behavior integration
- Improve accessibility and semantics

**Proposed API Direction**

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**Performance Opportunities**

- Reduce action re-renders
- Improve animated scroll performance
- Reduce style recalculation

首先阅读当前的 AppBar 实现和公共 API，然后将其行为与列出的 MD3 顶部应用栏变体和操作组合方向进行比较。在处理滚动行为、无障碍和渲染性能之前，先定义受影响的 API 和组件边界；完成的标准是这些变体和操作能够在各个平台上以一致的方式工作，并达到所述的性能目标。

由索引模型根据 Issue 内容生成。