A part's merge that comes back no longer blocks the split

• /flow:work:run with why = merged and the merge already recorded goes on: a run that recorded a part's merge and then stopped is relaunched with the same merged. It used to block with «nothing published»; it now takes the next startable MR/PR, or closes the work.
• /flow:feat:plan knows a split's parts may not reach the default branch: in unattended, a part cannot wait for an earlier one to be released or deployed. The plan merges the two, or drops the wait and leaves the real-release check to a person on the MR/PR.