feat: add accessibility adaptation layer

[adrcotfas]

Motivation

The library's existing reduce-motion support set animation.scale = 0 -- a v2-era flag that components had to explicitly check, with no connection to the MD3 theme.motion spring and duration tokens.

This PR wires OS accessibility into the theme properly. useAccessibleTheme replaces the inline PaperProvider logic, subscribes to reduceMotionChanged, and when active collapses theme.motion to instant springs and zero durations via the new reducedMotion preset. A clean theme.motion.prefersReducedMotion: boolean flag gives components a single readable signal instead of the magic scale === 0 check.

animation.scale is kept and still set to 0 for backward compatibility with components that haven't migrated yet. It is now formally deprecated in favour of theme.motion.prefersReducedMotion.

Two further deprecations are added while here: theme.mode ('adaptive' | 'exact'), which is an MD2 elevation-overlay concept superseded by tonal surface colors in theme.colors.elevation.*; and animation.defaultAnimationDuration, which was never read at runtime.
No component behavior changes. The reduce-motion path is functionally equivalent to before for all existing components. theme.motion.prefersReducedMotion and the reducedMotion preset are foundation plumbing -- they will be consumed when components migrate to theme.motion tokens in the per-component PRs that follow.

Related issue

Related issue
See https://www.notion.so/callstack/React-Native-Paper-Foundation-for-MD3-Expressive-34c5d027c0f880edba3df107cd35946f?source=copy_link

Merge order:

• refactor: remove MD3 prefix and use deprecated aliases #4910
• refactor: move styles/ into src/theme/ tree #4915
• refactor: add color role table and buildScheme #4916
• feat: add theme.state opacity tokens and split theme types #4917
• feat: add theme.shapes and cornersToStyle #4919
• feat: add emphasized typescale and fix letter-spacing spec bugs #4920
• feat: add motion tokens (spring, easing, duration) #4922
• feat: add elevation tokens and fix Android dp spec bug #4923
• feat: add accessibility adaptation layer #4924

Test plan

• yarn typescript -- no new type errors
• yarn test -- all tests pass

[callstack-bot]

Hey @adrcotfas, thank you for your pull request 🤗. The documentation from this branch can be viewed here.

[github-actions]

The mobile version of example app from this branch is ready! You can see it here.