Add ScrollView support

[Kyle-Ye]

Summary

Add public ScrollView support so OpenSwiftUI can render and interact with scrollable vertical and horizontal content.

Scope

• Add the public ScrollView container API and common initializers.
• Integrate with existing scroll-related plumbing such as scroll position, scroll targets, transactions, and content offset behavior.
• Support basic viewport layout, content sizing, indicators, and user-driven scrolling.
• Add examples or tests covering simple vertical and horizontal scroll views.

Acceptance criteria

• A basic ScrollView { ... } renders content larger than the viewport.
• Vertical and horizontal scrolling work in supported renderers/hosts.
• Content identity and layout remain stable across updates.