feat: add x402 core models and payment extensions

[nkavian]

Summary

• Reuse upstream x402 Python models and provide InFlow payment identifier and sponsorship declarations.
• Preserve decimal precision and caller-owned extension data.
• Document wire usage and upstream serialization behavior.
• Exercise shared Core vectors, Node-derived failure cases, and an isolated x402-only wheel consumer.

Verification

• make sync and make verify passed on Python 3.11, 3.12, 3.13, and 3.14.
• Shared fixtures match inflow-specs unchanged.
• Buyer workflows and Seller/facilitator network integration remain separate queue tasks.