Docs: clarify Cue usage in sliver-based scroll layouts

[Copilot]

The issue asked how Cue should be used with sliver widgets in CustomScrollView. This PR adds explicit guidance so users know where Cue should be applied in sliver trees.

• README: sliver usage guidance

  • Added a concise note in the trigger/usage section:
    • Animate the non-sliver widget rendered by each sliver (e.g. SliverToBoxAdapter child, SliverList item).
    • For SliverAppBar header animation, use CueFlexibleSpaceBar.

• Scope

  • Documentation-only change (README.md), no API or runtime behavior changes.

CustomScrollView(
  slivers: [
    SliverToBoxAdapter(
      child: Cue.onScrollVisible(
        child: Actor(
          acts: [.fadeIn(), .slideY(from: 0.1)],
          child: const Text('Animated box child'),
        ),
      ),
    ),
    SliverAppBar(
      expandedHeight: 200,
      flexibleSpace: CueFlexibleSpaceBar(
        title: const Text('Title'),
        background: Actor(acts: [.fadeIn()], child: Placeholder()),
      ),
    ),
  ],
)