One batched conversion-lift read against exactly one target: groups
(content-hash-keyed variant groups, read from the precomputed grades),
campaign_stage_ids (live per-stage cohort vs the stage's held-back
arm), campaign_ids (the same live read pooled across every stage of
the campaign — first-touch anchors campaign-wide, sent-wins across
stages), or campaign_stage_variant_ids (the stage read at per-arm
grain: the arm's sent recipients vs the whole stage's pooled held-back
arm). Response keys are the group keys / the stage id strings / the
campaign id strings / the stage-variant id strings; all map to the same
ConversionLiftEntryInfo list. Providing more or fewer than one target
is rejected. region_ids narrows the groups read to grades on the
selected regions' member states (union); it is rejected with the campaign
targets — campaign sends carry no experiment instance, so a region scope
cannot attribute them.