refactor(segmented-buttons): improve state model and rendering performance

Refactor SegmentedButton 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 selection APIs
- Improve density and layout handling
- Improve MD3 shape and state layers
- Simplify internal selection logic

**Performance Opportunities**

- Improve rendering in dynamic lists
- Memoize button state calculations
- Reduce unnecessary prop spreading

该 issue 将 SegmentedButton 确定为入口点，但没有指定文件、测试或具体的 API 设计。首先梳理该组件当前的实现和测试，然后解决选择、密度、布局、形状、state-layer 和性能要求；完成标准是就现代化 API 达成一致，并覆盖所列行为。

由索引模型根据 Issue 内容生成。