[A11Y-407] Smart contrast research prototype #63

Research opt-in measured contrast assistance for eligible visible components with compositing/cache, preserving gradients/focus/hover/controls/images and reporting unsupported elements; no automatic WCAG claim.

Acceptance: Performance, fixture evidence, restoration and owner promotion decision exist.

Master guidance: https://github.com/prabothcharith/accessibility-react/blob/main/docs/DEVELOPMENT.md.