JSON canonicalization — canonicalize structured data with RFC 8785 JCS; return canonical JSON text, UTF-8 byte length, and SHA-256 hash for integrity checks, signature preparation, and deduplication. Includes an EIP-712 signed attestation. Use when an agent needs an idempotency key, cache key, dedup hash, or verifiable commitment. $0.002 per call. Free verification at POST /v1/verify.