feat: implement x402 buyer payment workflows

[nkavian]

Summary

• Add x402 Buyer selection, InFlow-managed signing, two-phase approvals, polling, cancellation, and completion hooks.
• Compose with upstream HTTP/MCP clients and external EVM/Solana signing schemes; include opt-in EIP-7702 sponsorship.
• Document payment-policy scope, approval ownership, signer lifetimes, and upstream Python's blocking Solana calls, tracked in Python async client blocks the event loop during Solana payment creation x402-foundation/x402#3649.

Verification

• make sync and make verify on Python 3.11, 3.12, 3.13, and 3.14.
• Shared Buyer contract fixtures, concurrent cancellation and refresh failures, real upstream HTTP/MCP composition, and external EVM signing.
• Permit2 sponsorship composition and signatures compared with independently generated Node vectors.
• Built distributions installed into isolated base, x402-only, MPP, and all-extras consumers.

No live payment or blockchain transaction was executed. Hosted sponsorship requires an environment exposing the corresponding preparation endpoint.