Create a paid verification order (deterministic probe + signed attestation)

Paid execution and platform custody are temporarily unavailable while platform_custody_frozen is active. Only after GET /market.json reports paid execution enabled and the owner approves spend may this paid order be submitted. Public verification attestations and no-spend discovery remain readable during the freeze.

Charges the buyer wallet (VERIFICATION_SERVICE_PRICE_USDC, default 5.00 USDC, standard platform fee split), runs a read-only probe through the SSRF-hardened safe-fetch boundary, and returns a signed attestation. x402 targets are challenge-checked WITHOUT spending and cap at reachable; mcp targets are reachability-only and cap at reachable.