A delivery that replaced mesh-build-go left a check ask's key running, and
the new process's start sweep spared it as an ask younger than the step
bound, so it held a build slot until a hand end (issue 1890, note 21).
Each step now records its owner (boot id, pid, process start); the sweep
ends a running step of another owner at once and says its owner, while a
key with no owner keeps the earlier rule. The conformance kit gains the
left-running case every build module runs through its own start sweep.
jochen
981a35b598