cookbook: add xProof blockchain certification middleware for AI SDK

[jasonxkensei]

This PR adds a cookbook example showing how to use xProof middleware with the Vercel AI SDK to automatically certify every AI generation on the MultiversX blockchain.

The middleware wraps streamText / generateText transparently — zero changes to existing application code.

What it demonstrates

• Drop-in xproofMiddleware() wrapping streamText and generateText
• Automatic SHA-256 hashing of prompts + outputs (no PII sent)
• Batch mode for high-throughput applications
• shouldCertify filter to certify only production-critical generations

Install

npm install @xproof/xproof

Checklist

• Works with App Router route handlers
• Works with streamText and generateText
• TypeScript types included
• Free tier available (10 certs/day)