fixup-features in CI

maint/fixup-features does two logically slightly separate things:

1. Checks that every feature is part of experimental or full (or non-additive)
2. Maintains the two mapping directions (full => feat; feat => __is_full) in sync

The latter generates a lot of diff noise. I think it's OK to defer that work until the release since that'll reduce conflicts during normal development.

But the former ought IMO to be done by the authors of MRs.

We should make fixup-features have a mode where it identifies missing information and fails iff there is any, and run it in CI.

I think, looking at the output, that one option would be: run fixup-features and see if the output contains any hunks which introduce XXXX. @nickm?