Add changelog entries for upcoming release with new features, fixes, and refactors

This updates the CHANGELOG.md to include detailed entries for the upcoming
unreleased version. It adds new features for iOS, macOS, and SwiftUI, updates
existing behaviors for improved layout and rendering performance, and fixes
numerous bugs related to text view sizing, scrolling, and rendering quirks. Also
includes internal refactors and platform/toolchain updates to improve
development workflow and API consistency.