Optimize adaptive preview GIF

[mikonyaa]

Summary

• recompress the Adaptive App Shell showcase GIF from 6,823,862 bytes to 4,387,038 bytes
• keep the preview at 400×870, 20 FPS, and 258 frames
• update the recording script so future captures use the same lighter GIF encoding
• add changelog entry for 1.0.2

Validation

• swift test
• swiftlint
• xcodebuild -project Examples/AdaptiveShellDemo/AdaptiveShellDemo.xcodeproj -scheme AdaptiveShellDemo -destination "generic/platform=iOS Simulator" CODE_SIGNING_ALLOWED=NO build
• ffprobe dimensions/frame-rate/frame-count check