Improve bounds-change handling for bottomToTop layout

[bryankeller]

Details

This PR improves some bounds-change handling for the bottomToTop layout. The math is messy, but it works. There might be a deeper truth here that would simplify the calculation so that we don't need a different case for shrinking / growing bounds, and the weird clamp we need to do, but I haven't found it yet.

Simulator.Screen.Recording.-.iPhone.16.Pro.-.2025-04-16.at.21.04.47.mp4

Motivation and Context

Fixes an issue where a bounds change caused the collection view to not preserve the bottom visible item properly.

How Has This Been Tested

Example app.

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.