RFC 8785 JCS canonicalization + ES256 signing implementation

[joelteply]

Summary

Implement the actual signing and verification logic. Currently write-only — attestations can be created but not verified.

Requirements

• RFC 8785 (JSON Canonicalization Scheme) implementation for deterministic payload serialization
• ES256 signing: signature.value = ES256(SHA-256(JCS(payload)))
• verify_signature() function in all three languages (Rust, Python, TypeScript)
• Canonical payload builder that extracts the signable fields from an IntegrityAttestation
• Timestamp validation (ISO 8601 format, not in future, max age enforcement)

Blocking

This blocks all signed attestation work (Phase 2+). Without canonicalization, cross-language verification breaks.

References

• RFC 8785
• docs/ATTESTATION.md — Canonicalization section