LAB: nightly train lane cleanroom-cpu (off the release verdict) #5151

The nightly train's cleanroom-cpu lane moves from the release verdict to LAB (#5147 PR). It has no last green and no measured duration on main.

• Owner: aprender-cd
• While LAB, a red or not_measured night updates this ticket and stays silent. The lane never stops a merge or a release.
• After 7 days red or unmeasured, the lane stops running until its owner brings it back.
• It returns to the verdict only with an operator yes, after three green nights on main, with one gate retired for each one added.
• The tag-time step this lane rehearses stays on the release path unchanged.

keep-open: LAB ticket for the lane; closed by its owner when the lane is retired or returned to the verdict.