[A11Y]: Define keyboard and VoiceOver behavior for diff navigation #70
Description
Problem
A future diff workflow will be incomplete if change navigation is only visually clear but not accessible to keyboard and assistive-technology users.
Proposed solution
Track accessibility behavior for diff view explicitly, including focus order, change-region traversal, source labeling, and spoken context.
Acceptance criteria
- Keyboard users can move between change regions
- VoiceOver exposes which side and which file each section belongs to
- Focus never gets trapped in the compare surface
- Diff navigation works on macOS and iPad keyboard workflows
- Accessibility expectations are defined early enough to shape implementation