The strict corpus guard requires every listed consumer of the update-marker
corpus to exist. On this stacked PR the Electron reader lands in #132345 and
the two hand-off script readers in #132354, which merge before this PR, so
the guard went red here although the composite has all five.
Absence is now admitted only for consumers named in _LANDS_IN_SIBLING_PR
with the PR that lands them. A missing consumer no sibling lands, a fixture
whose every listed consumer is absent, and an allowance entry that names no
listed consumer all still fail. Once the siblings are on main the table can be
emptied and the full strict check applies unchanged.
(cherry picked from commit cfdfa6beeb4a1d8f17eb0ef7b078547b3c39cd5d)
(cherry picked from commit 3fb57d4a4e8805da89338ac0f469924171bcc0d1)
ef2f37fa57