feat: Native implementation for listeners (also now fires for AppClip/extension changes)

[mrousavy]

Summary

• Register a singleton native MMKVHandler during MMKV initialization and forward MMKV logs through Nitro's logger.
• Replace the value-only native registry with a per-MMKV-ID listener registry that routes both local key-level changes and native outer-process content changes through the existing addOnValueChangedListener(...) API.
• Add checkExternalContentChanged() to the MMKV HybridObject, with web/mock no-ops.
• Conservatively fan out external content changes to all current and previously known keys, since upstream MMKV only reports the mmap ID and not the changed key.
• Refresh external changes on app foreground by calling checkExternalContentChanged() from the existing AppState listener setup.
• Add docs and Harness coverage for the new check API.

Verification

• bun run specs
• bun run lint-cpp
• bun lint-ci
• bun run typecheck
• bun run test -- packages/react-native-mmkv/src/__tests__/hooks.test.tsx --runInBand
• HARNESS_WEB_URL=http://localhost:3000 bun --cwd example test:harness __tests__/MMKV.harness.ts --harnessRunner web
• cd example/android && ./gradlew :react-native-mmkv:externalNativeBuildRelease --no-daemon

Note: earlier local bun --cwd example build:android-release passed the react-native-mmkv native C++ build for all ABIs, then failed later in :app:createBundleReleaseJsAndAssets while starting the local node_modules/react-native/sdks/hermesc/osx-bin/hermesc binary. The previous CI run passed Android build, iOS build, Android Harness, Web Harness, and all fast checks before this API simplification; CI is rerunning for the amended head.