`ensureJsonlGenerationCurrent` returned a live Zstandard body iterator that
every production caller disposed unread before decoding the same bytes again;
the header read now decodes only the first frame and owns no decoder past its
return. The JSONL backend reuses the generation module's stable-revision read
instead of keeping a second copy, `encodeCurrent` encodes the validated
current artifact it is documented to receive instead of re-running the
migration chain on it, and the test-only `inspectVersion` catalog method and
the `migrateSessionSnapshotFixture` duplicate of
`prepareSessionSnapshotFixtureForComparison` are removed.
Tianyi Cui