feat: Add Bitcoin Cashu support

[yashbhutwala]

Summary

• add NUT-24 cashu-token scheme with bitcoin mainnet/testnet defaults and multi-mint metadata
• wire facilitator to validate requirements, check unspent proofs, and settle via CashuWallet.receive
• update TypeScript/Python SDK types, docs, and focused tests for interoperable Cashu support
• Sister PRs: https://github.com/google-agentic-commerce/a2a-x402/pull/20, https://github.com/google-agentic-commerce/AP2/pull/24

[cb-heimdall]

🟡 Heimdall Review Status

Requirement	Status	More Info
Reviews	🟡 0/1	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 2 if repo is sensitive 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 1 Sum 2

[vercel[bot]]

@yashbhutwala is attempting to deploy a commit to the Coinbase Team on Vercel.

A member of the Team first needs to authorize it.

[cloudsupper]

Awesome!

[apmcdermott]

Hi @yashbhutwala! Thanks for the contribution. We're in the process of implementing v2 (discussion here), which will likely make your Cashu integration process much simpler.