DESCRIPTOR_
The descriptor format this library writes, and the one it understands.
Major is the compatibility boundary: a descriptor with a different major version is refused on read, because a changed major means a field this reader would misinterpret rather than merely fail to see. Minor is additive — a descriptor written by a later minor version of the same major reads correctly here, losing only the fields this version has no name for, which is what ignoreUnknownKeys is for.
It is the schema's version, not the library's and not the model's, so it moves only when the descriptor's shape moves.