CI cancel-in-progress hides compile errors when multiple agents push

With GitHub Actions concurrency cancel-in-progress on a branch, several agents pushing in quick succession cancel each other's builds, so a commit can go several pushes without ever finishing a CI build. A compile error introduced early (here Swift's 'passing non-escaping parameter to function expecting an @escaping closure') then surfaces on a later push and looks like that pusher's fault. Serialize pushes: each pusher waits for the previous run to go green. When a run goes red, git blame the failing line on the remote branch to route the fix to the real owner.