v0.1.0

Added

• SkeletonContainer(loading) — hosts one shared shimmer animation per subtree.
• Modifier.skeleton(shape) — draws a placeholder matching the element's own
  measured bounds while loading, crossfading to real content when it's done.
  Supports SkeletonShape.Auto (default), SkeletonShape.Circle, and
  SkeletonShape.RoundedCorner(radius).
• SkeletonDefaults — corner radius, MaterialTheme-derived shimmer colors,
  and crossfade animation defaults, all overridable per SkeletonContainer.
• Android, iOS, and Desktop (JVM) targets.