MIME-side helpers used by the S/MIME generators and parsers in
org.bouncycastle.mail.smime: body-part output canonicalisation,
file-backed data sources for large attachments, and CRLF normalisation.
Higher-level S/MIME signed-message validator that runs structural, signer, and
certificate-path checks against a SignedMailValidator.ValidationResult and
reports failures as localised messages through ErrorBundle.