0392 — cursor claimed advances via GREATEST so a group running both paths cannot regress its frontier

Context. With LIFECYCLE groups now holding cursor rows, a group mistakenly running both the cursor-claim path and the FanOut path would have two writers to the same claimed column, and a plain assignment from the slower path could move the frontier backward into territory the other path already claimed — reopening ranges for overlapping delivery.

Decision. claimed advances via GREATEST, so no write can regress it below its current value regardless of which path issued the write.

Consequences. Misconfiguration (both paths active on one group) degrades to redundant work instead of double-delivery overlap. The invariant is monotonic: the claim frontier only moves forward.