t27b-lab: a lane request skips the fuzzer (7 of 22 minutes per run) #8267

Problem

Every lane PR under #6063 / #6198 waits for a signed run of its head on t27b-lab (/srv/requests/<sha>, #7672). The master run of 0fca4bd took 22 minutes, and its fuzz step took 427 s of that (corpus 341 s, reference 328 s, receipt 126 s).

A lane request is judged by corpus-receipt compare on its receipt; the fuzzer's summary is not part of the receipt and no lane reads it. Master runs keep the fuzzer: they feed the ratchet.

Change

contrib/railway/t27b-lab/lab.py: the fuzz step runs only when master is true. One line edited, net 0 hand-written lines (#8236).

Acceptance

• A request run's JSON has no fuzz step; a master run still has it.
• The lab is redeployed with railway up from a clean git archive of contrib/railway/t27b-lab after the merge.