refactor(appbar): improve MD3 variants and action composition

Refactor AppBar 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 support for MD3 top app bar variants
- Simplify action rendering
- Improve scroll behavior integration
- Improve accessibility and semantics

**Proposed API Direction**

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**Performance Opportunities**

- Reduce action re-renders
- Improve animated scroll performance
- Reduce style recalculation

Start by reading the current AppBar implementation and public API, then compare its behavior with the listed MD3 top app bar variants and action composition direction. Define the affected API and component boundaries before addressing scroll behavior, accessibility, and rendering performance; done means the variants and actions work consistently across platforms with the stated performance goals.

Written by the indexing model from the issue text.