Hash the identity-bearing fields of a `Delta` with blake3 and truncate to 128 bits. A [`RegistryDelta::Merge`] is addressed by its sorted parent set alone (author and timestamp excluded), so two peers merging the same tips mint the identical `Rev` and dedup. Every other delta hashes `(parent, author, timestamp, kind)`.
(parent: Option<Rev>, author: PeerId, timestamp: TimeStamp, delta_type: &RegistryDelta)
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