chore: upgrade to Expo SDK 56 and React Native 0.85.3

[lodev09]

Summary

Upgrades the monorepo to Expo SDK 56 / React Native 0.85.3.

• example/expo: SDK 54 → 56 (RN 0.85.3, react 19.2.3, expo-router 56, TypeScript 6)
• example/bare + root devDeps: RN 0.83 → 0.85.3, react 19.2.3, @react-native/* 0.85.3
• root @expo/config-plugins: ^9.0.0 → ^56.0.8

Breaking changes handled

• Jest preset moved out of react-native → @react-native/jest-preset (root + bare jest config)
• Expo splash config removed → migrated to the expo-splash-screen plugin
• Dropped now-default newArchEnabled
• _layout.tsx uses expo-router <Stack /> (the forked react-navigation theme types collided with bare's real @react-navigation/native in the shared tsc program; app is light-only so behavior is unchanged)
• Added expo-router peers: expo-constants, expo-linking, react-native-safe-area-context
• bare Gemfile: added gem 'nkf' (provides kconv, removed from Ruby 3.4 stdlib) — matches the RN 0.85 community template; no Ruby downgrade needed

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update

Maintenance / dependency upgrade (chore). No public API change.

Test Plan

• yarn typecheck ✅
• yarn test ✅
• expo-doctor → 20/21 (only the monorepo same-version duplicate false positive)
• Native builds (Debug):
  • bare iOS ✅ (xcodebuild + react-native build-ios end-to-end)
  • bare Android ✅
  • expo iOS ✅
  • expo Android ✅

Screenshots / Videos

N/A

Checklist

• I tested on iOS
• I tested on Android
• I tested on Web
• I updated the documentation (if needed) — no component/API changes