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

[ruben-rebelo]

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