feat(examples): add DA payment anchoring client example

[zse4321]

Adds a TypeScript client example demonstrating how to anchor x402 payment decisions externally using Decision Anchor.

What it does

Wraps @x402/fetch payment flow with DA Decision Declarations:

1. Before payment — creates a DD recording the authorization scope
2. x402 payment — executes USDC payment via wrapFetchWithPayment
3. After payment — confirms the DD, anchoring completion timestamp

This provides external, append-only proof of what was authorized, at what scope, at the moment of decision — independent of on-chain records and internal logs.

Files

• examples/typescript/clients/da-anchoring/index.ts — main example
• examples/typescript/clients/da-anchoring/package.json — deps (@x402/fetch, @x402/evm, decision-anchor-sdk)
• examples/typescript/clients/da-anchoring/README.md — setup guide
• Config files (.prettierrc, eslint.config.js, tsconfig.json) copied from clients/fetch

No SDK changes — examples only (private package). No changeset needed.

Checklist

• Formatted and linted (prettier, eslint — 0 errors)
• Commit is GPG-signed
• No changelog fragment needed (examples-only, private package)