Sync format compatibility

Every durable or externally visible Sync format is registered in eng/sync-formats.json. A test binds the registry to implementation constants and named compatibility evidence.

Format	Current	Minimum readable	Upgrade policy
NATS envelope	1	1	Current integrity-checked binary envelope only; unknown versions fail closed.
OpenSearch storage	2	2	Current generation/control format only; incompatible changes require an explicit rebuild.
PostgreSQL Sync schema	2	1	Transactional in-place migration from v1; future versions fail closed without being rewritten.
Redis document	1	1	Current integrity-checked binary document only; unknown versions fail closed.
Redis storage	2	2	Current pipeline storage format only; incompatible changes require an explicit rebuild.
Transform fingerprint	1	1	Canonical length-prefixed SHA-256 input; the checked fixture prevents silent identity drift.

Before increasing a version, retain fixtures or a live migration test for every supported older version, add future-version rejection, update the registry, and document destination compatibility. Removing a readable version requires an explicit migration, rebuild, or replay boundary and a release note.