Stop Precompute Build

End a build that is not going to finish on its own.

A build whose process died clears itself once the lease it stopped renewing expires. This is for the other case, which nothing else resolves: a build still renewing its lease while making no progress, where the only other way out is the duration ceiling.

Nothing being served changes. A build stages under a new version key and only its last phase moves the published pointer, so stopping one leaves every pod on exactly what it was already serving.