Live format compatibility

Every durable or externally visible Live format is registered in eng/live-formats.json. A test binds the registry to implementation constants, the versioned protobuf package, and named compatibility evidence.

Format	Current	Minimum readable	Upgrade policy
gRPC contract	1	1	Protobuf v1; additive fields retain their numbers and incompatible changes require a new package.
Replay JSON	1	1	Current media type only; unknown event media types fail closed at the client boundary.
Resume token	1	1	Signed current version only; unknown versions cannot authenticate or resume.
PostgreSQL Live schema	2	1	Transactional in-place migration from v1; future versions fail closed without being rewritten.

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 client and storage compatibility. Removing a readable version requires an explicit migration or reconnect/reset path and a release note.