ApplicationMessageDigest

Content commitment of an application message body, version 1 (docs/application-delivery.md). A client keeps it with a committed inbound message so that a later message with the same logical ID but another body is detected instead of acknowledged. Local storage only: it is never sent and is not part of any wire format.

SHA-256( u32 length of domain | domain (UTF-8, ProtocolConstants.PROCESSED_MESSAGE_DOMAIN)
       | u32 length of body | body )

Integers are unsigned 32-bit big-endian. Frozen: the vectors in ApplicationMessageDigestTest must never change.