/// Everything a read needs to know about one canonical body without loading it.
///
/// The public `content_hash` cannot carry this weight alone. It is derived from
/// the event and the node identity, and a projection writer accepts a detail
/// directly, so a write can change the stored text without a reliable change to
/// that token. `record_digest` is computed over the exact stored record bytes at
/// write time and is the identity a card or an expansion may bind to.
///
/// Storage owns this. It is not a reader's claim and not a field of a card.