docs: add React Native integration guide

[0dillon]

Summary

This PR adds the React Native integration guide for wiring up @wraith-protocol/sdk and @wraith-protocol/sdk-react within mobile applications (both Expo and Bare React Native workflows).

Closes #76

What Changed

• Added a new integration guide at guides/integrations/react-native.mdx.
• Documented cryptographic shimming of global.crypto.getRandomValues using expo-crypto (for Expo) and react-native-get-random-values (for Bare React Native workflow).
• Documented polyfilling Buffer for React Native.
• Provided a standard wrapper pattern for persistent storage utilizing @react-native-async-storage/async-storage.
• Added a complete TS/TSX boilerplate component (MainScreen) illustrating connection, stealth payment sending, and incoming payment scanning.
• Registered the new guide under the "Guides" navigation group in docs.json.

Key Design Decisions

• Storage Interface: Formulated the AsyncStorage adapter to conform to standard Key-Value storage interfaces (getItem, setItem, removeItem) expected by custom React/React Native SDK initializations.
• Top-Level Shimming: Placed shims at the very beginning of the entry file (e.g., App.js / index.js) to guarantee secure random bytes are available before any cryptographic SDK packages are imported.

Acceptance Criteria Checklist

• Snippet CI passes (npm test compiles all MDX code blocks successfully).
• Clear instruction distinction between Expo vs. Bare workflows.
• Step-by-step wiring notes for AsyncStorage + expo-crypto.
• Full, copy-pasteable send + receive TSX code example.
• Navigation sidebar entry added to docs.json.

Test Output

Ran local snippet checks against all workspace documents:

> npm test

MDX files scanned: 38
Code fences found: 438
Checked snippets: 425
Skipped no-check snippets: 13
Snippet check passed.

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[truthixify]

Merged. RN guide fills a real gap for mobile devs, thanks.