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The Assertion proof: a detached author signature over a ContentId.
It answers who vouched for this object? and lives beside the node,
never inside its hashed bytes (see crate::authority, proof table). The
signed message is a domain tag plus the CID’s own byte form — no bespoke
canonical encoding. The assertion does not carry the CID: the reader
supplies the CID they computed, so a signature cannot vouch for anything
but what the reader actually holds.