Need to address protobuf versioning

If we allow customers to deploy their own worker with a "locked" protobuf version, what if the Host makes a protobuf change?

Current we require host + worker + protobuf version to be compatible.

The issue does not name any files, tests, or entry points. Start by tracing how the host and customer workers negotiate or validate protobuf compatibility, then clarify the expected behavior when their versions differ. Done requires an agreed versioning policy and corresponding implementation scope.

Written by the indexing model from the issue text.