[lazerg]

Summary:

Appearance.setColorScheme() updates the cached scheme that getColorScheme() returns, but it never emits change. So useColorScheme() keeps the old scheme until the native appearanceChanged event arrives, and the same screen can show both schemes. Now setColorScheme() emits change when the scheme actually changes, and the native handler skips values that match the cache, so listeners don't get the same change twice. 'auto' still updates when the native event arrives, because the system scheme isn't known in JS yet at that point.

Fixes #59946

Changelog:

[General] [Fixed] - Appearance.setColorScheme() now notifies change listeners, so useColorScheme() updates right away

Test Plan:

Added Appearance-itest.js with a fake native module. It checks that setColorScheme('light') calls the listener once, that the native echo doesn't call it again, and that a component using useColorScheme() re-renders with light in the same task. I couldn't build the Fantom runner locally, so I checked the same cases under plain Jest: they fail on main and pass with the fix. yarn test packages/react-native/Libraries passes, and Flow, ESLint and Prettier are clean.