[A11Y-101] Typed preferences registry and state reducer #32

Implement validated single-source settings and feature definitions for typography, contrast, links/focus, motion, images, fonts, cursor/reader/saturation plus locale/profile/audio/experimental modules. Model base + profile snapshot + override map and original/null values.

Acceptance: Tests cover bounds, normalization, effective precedence, atomic updates and stable IDs.

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