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

L’issue identifie SegmentedButton comme point d’entrée, mais ne nomme aucun fichier, test ni conception concrète de l’API. Commencez par cartographier l’implémentation actuelle et les tests du composant, puis clarifiez les exigences de sélection, de densité, de mise en page, de forme, de state-layer et de performance ; le travail sera considéré comme terminé lorsqu’un accord aura été trouvé sur l’API modernisée et que les comportements listés seront couverts.

Rédigé par le modèle d'indexation à partir du texte de l'issue.