Artifacts are immutable and content-addressed

An immutable artifact has no invalidation semantics at the store: nothing to revalidate, no negative cache, no single-flight. (Field-level revalidate is data freshness inside an artifact, not artifact invalidation.) Publishing points at a new hash; rolling back points at the old one.

The only mutable output is a route pointer, one atomic record per route. A single manifest document was the first design and it permitted silent lost updates: two concurrent publishes read the same snapshot and the second overwrote the first, with no error. A single-key write is atomic on object storage, a database row, or a filesystem; a read-modify-write over a whole table is not.