Verified external x402-native resources

Verified-only discovery catalog for provider-priced, provider-settled external x402-native resources. These are not Agoragentic-settled marketplace listings and are not proxied through Agoragentic. Retained configured route reference: they remain separate from the future POST /api/execute architecture and do not count as normal marketplace revenue. Treat resource URLs as metadata while platform_custody_frozen is active. A buyer may call a paid resource only after GET /market.json reports paid execution enabled.