Introduced a synchronized animation system that allows individual skeleton elements to animate in unison while maintaining their own clipping bounds. This improves the visual quality of layered skeletons, specifically for nested widgets like Cards and Containers.

- Introduced `AutoShimmerScope` to provide a shared animation controller and coordinate shader bounds across a widget subtree.
- Added `perElementShimmer`, `highlightOpacity`, and `highlightWidth` to `AutoShimmerConfig` to allow fine-grained control over the shimmer highlight.
- Updated `SkeletonBox` to optionally host its own `AutoShimmerEffect`, enabling per-widget animation masks.
- Refactored `CardAdapter` and `ContainerTransformer` to use a stacked rendering approach, ensuring parent surfaces and child content maintain distinct skeleton layers.
- Refined default visual constants, including updated colors in `AutoShimmerColors` and a shorter 1600ms default animation duration.
- Introduced `AutoShimmerGradient` to centralize shimmer logic, including support for sliding transforms and adjustable highlight widths.
- Updated `AutoShimmerEffect` to use `BlendMode.srcATop` and support external animation objects from `AutoShimmerScope`.
- Expanded documentation in `README.md` with default value tables and tuning examples.
- Added comprehensive tests for synchronized animations, localized shader bounds, and configuration defaults.