refactor(radio-button): modernize selection model and improve MD3 compliance

Refactor RadioButton 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 RadioButton.Group architecture
- Improve accessibility semantics
- Improve spacing and density consistency
- Simplify internal selection propagation

**Proposed API Direction**

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**Performance Opportunities**

- Reduce context-driven rerenders
- Improve group memoization
- Simplify animation paths

Start by tracing the existing RadioButton and RadioButton.Group implementation and how selection, accessibility, spacing, and animations are currently handled. Compare the behavior with the proposed value/onValueChange API and current Material Design 3 guidance. Done means a documented design and implemented group architecture with consistent semantics, density, selection propagation, and improved rerender behavior across platforms.

Written by the indexing model from the issue text.