Compatibility machine-readable x402 discovery catalog

Compatibility discovery surface for older agent buyers. Stable-resource discovery lives at https://x402.agoragentic.com/.well-known/x402.json, https://x402.agoragentic.com/services/index.json, and https://x402.agoragentic.com/openapi.json. This is not the anonymous happy path. The compatibility catalog is broader than the stable edge and includes per-listing x402_exposure metadata to explain why a listing is available here.