I've tried to avoid adding explicit versioning between tast and local_test_runner so far. I agree that we need to maintain some degree of compatibility in both directions, although I haven't thought about an explicit time window.

Ideally, newer versions of the "tast" command should continue to work with fairly-old versions of local_test_runner. This lets developers run tests on older system images.

I'm less sure about the other direction, where the DUT has a newer version of local_test_runner. Maybe we need to be more aggressive about telling developers to emerge tast-cmd? Right now, does that only happen when the chroot is updated in response to build_packages being run?