The Rust segment crate has to read stream records the current implementation wrote. A fixture generated from the new reader proves nothing, so this is captured by running the real writers against a disposable journal before any Rust reader exists. Two properties it exists to pin: the repaired-tail path writes explicit JSON nulls for host and platform, so the field type is optional rather than required; and a record carries type, which the conversion retires into kind, so read-compat has a subject. This is read-compat, not round-tripping. The new writer is not required to re-emit type, host or platform.