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

El issue identifica SegmentedButton como el punto de entrada, pero no especifica archivos, pruebas ni un diseño concreto de la API. Primero hay que mapear la implementación actual y las pruebas del componente, y después resolver los requisitos de selección, densidad, diseño, forma, capa de estado y rendimiento; se considerará terminado cuando exista acuerdo sobre la API modernizada y cobertura para los comportamientos enumerados.

Escrito por el modelo de indexación a partir del texto del issue.