feat: add custom paymentFetch option to proxy

[matiasedgeandnode]

Summary

• Adds optional paymentFetch parameter to startProxy() for custom payment handlers
• Enables integration of alternative payment systems (spend limits, budget enforcement, etc.)
• Falls back to built-in x402 payment handler when not provided

Test plan

• Verify existing behavior unchanged when paymentFetch is not provided
• Test custom paymentFetch function is called when provided

🤖 Generated with Claude Code

[VickyXAI]

Thanks Matias — closing as superseded rather than rejected.

The stated motivation (spend limits, budget enforcement) is now native: main ships src/spend-control.ts with a full SpendControl class covering perRequest/hourly/daily/session windows with file and in-memory storage. And the diff no longer applies — it patches ProxyOptions around line 814 (now ~1308) and calls createPaymentFetch(), which was deleted in the @x402/fetch rewrite.

For the record on design: options.paymentFetch ?? createPaymentFetch(...) is a total-replacement seam on the money path — caller code could sign anything, to anyone, for any amount. If a hook is still wanted, #205's abort-only beforePayment is the safer shape.