feat: add modular resource fetcher adapters for Expo and bare React Native

[rizalibnu]

Description

This PR introduces modular resource fetcher adapters to support both Expo and bare React Native environments, replacing the previous monolithic approach with a flexible, platform-specific architecture.

Key Changes
New Adapter Packages:

• @rn-executorch/expo-adapter: Resource fetcher for Expo projects using expo-file-system and expo-asset
• @rn-executorch/bare-adapter: Resource fetcher for bare React Native projects using @dr.pogodin/react-native-fs and @kesha-antonov/react-native-background-downloader

Initialization Changes:

• Added initExecutorch() function that requires explicit adapter selection
• Users must now choose and configure the appropriate adapter for their project type
• Provides better separation of concerns and platform-specific optimizations

Documentation Updates:

• Created individual README.md files for each adapter package

Introduces a breaking change?

• Yes
• No

Migration Required:
Users must now explicitly initialize the library with a resource fetcher adapter:

// Before (no initialization needed)
import { useLLM } from 'react-native-executorch';

// After (required initialization)
import { initExecutorch, useLLM } from 'react-native-executorch';
import { ExpoResourceFetcher } from '@rn-executorch/expo-adapter'; // or BareResourceFetcher

initExecutorch({
  resourceFetcher: ExpoResourceFetcher,
});

Type of change

• Bug fix (change which fixes an issue)
• New feature (change which adds functionality)
• Documentation update (improves or adds clarity to existing documentation)
• Other (chores, tests, code style improvements etc.)

Tested on

• iOS
• Android

Testing instructions

For Expo projects:

• Install dependencies: yarn add @rn-executorch/expo-adapter expo-file-system expo-asset
• Initialize: initExecutorch({ resourceFetcher: ExpoResourceFetcher })
• Run existing LLM example app to verify model downloads work correctly

For bare React Native projects:

• Install dependencies: yarn add @rn-executorch/bare-adapter @dr.pogodin/react-native-fs @kesha-antonov/react-native-background-downloader
• Initialize: initExecutorch({ resourceFetcher: BareResourceFetcher })
• Run the bare React Native example app in PR feat: add bare React Native LLM chat example app #763

Note: A separate PR will add a dedicated bare React Native example app to make this PR easier to review. The Expo example apps can be used to verify the Expo adapter functionality.

Screenshots

Related issues

Closes #549

Checklist

• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have updated the documentation accordingly
• My changes generate no new warnings

Additional notes

Why This Change:

• Different React Native environments have different filesystem APIs and c
• apabilities
• Expo projects benefit from using Expo's managed filesystem APIs
• Bare React Native projects can leverage native libraries with background download support
• Modular architecture allows for better platform-specific optimizations
• Enables future extensibility for other environments (e.g., React Native Windows, macOS)

Split Into Multiple PRs:
To make review easier, this work has been split:

• This PR: Core adapter infrastructure and Expo adapter implementation
• Follow-up PR: Bare React Native example app demonstrating the bare adapter usage

BREAKING CHANGE: initExecutorch() with explicit adapter selection is now required before using any react-native-executorch hooks. Users must install and configure either @rn-executorch/expo-adapter or @rn-executorch/bare-adapter depending on their project type.

[msluszniak]

Lint CI fails (you don't need to worry about the second failing CI ;)). Please could you fix the errors from this CI?

[rizalibnu]

@msluszniak I’m not able to reproduce the lint CI failure locally — everything passes on my side.

I’ll take a closer look and investigate further to see what might be causing the discrepancy (environment, cache, or config differences). I’ll follow up with a fix or more details as soon as I find the root cause.

[msluszniak]

@rizalibnu Sure thing, maybe the configuration of the CI itself does not work with as it should. We'll also look at this, don't worry :)

[rizalibnu]

@msluszniak Found the issue 👍
CI was failing because react-native-executorch types come from lib/typescript, which isn’t built on a fresh run. It worked locally since I already had the build artifacts.

Fixed by adding a build step before adapter type checks and bumped Node to 22 in .nvmrc due to an ESM-only dependency (arktype via react-native-builder-bob).