ADR 0003: Serialization

Use strict versioned JSON, unpadded base64url bytes, RFC 3339 timestamps, and canonical authenticated metadata across persisted formats.

Status: accepted for v0.1.

Use strict versioned JSON schemas, unpadded base64url for bytes, UTC RFC 3339 timestamps, and JSON Canonicalization Scheme output for authenticated additional data. Reject unknown fields, invalid lengths, unsafe integers, malformed encodings, and unsupported versions before crypto.