LAB nightly ratchet comparand: refuse when base == HEAD; idle main must not fail the fetch #5204

Follow-up to #5182 / PR #5203, from seat 1's review (non-blocking there). Both are in the nightly comparand step for the shrink-only ratchets, which is LAB: it cannot block a merge or a release.

1. Idle main makes the fetch fail. The step fetches main with --shallow-since='3 days ago'. If main has no commit in that window, the fetch can fail with "no commits selected", and the nightly goes red for a reason that has nothing to do with growth. Fix: deepen by a fixed count as a fallback (or fetch by depth), and name the reason when no comparand is found.

2. The comparand can equal HEAD, which makes the ratchet vacuous. rev-list -1 --before='24 hours ago' returns HEAD itself when main has been idle for 24 h or more. The ratchet then compares HEAD with HEAD and passes having compared nothing. It must refuse instead: if the base sha equals HEAD, print comparand == HEAD and exit with the decline status (no comparison ran), never a pass.

Acceptance: a case table row for each of the two (idle main with nothing in the window; base == HEAD), RED before and GREEN after.

Refs #5182