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

The issue identifies SegmentedButton as the entry point, but names no files, tests, or concrete API design. First map the component's current implementation and tests, then resolve the selection, density, layout, shape, state-layer, and performance requirements; done requires agreement on the modernized API and coverage for the listed behaviors.

Written by the indexing model from the issue text.