Confusion over 'ineligible_participation' status

Nobody has claimed this yet.

## Description

When a participant starts a study (clicks "Participate now!") but doesn’t get as far as completing the consent frame, they’re counted as “participated” for the purpose of prior participation eligibility. However, researchers don't have any information about that response because it is un-consented. This it makes it difficult for researchers to determine why a participant is being marked as "ineligible_participation".

## Researcher POV

"When I see that a response is marked "ineligible_participation", I should be able to figure out why exactly it has that status. I need more information about their prior response(s), including which study(ies) and how far through them they got, in order to decide whether I can include their response in my anaysis."

We could consider providing minimal info about study responses that are collated by child ID, regardless of consent approval. E.g. just a timestamp and how far through the study they got ("sequence"). However the easiest way to do this would be on the single study level. So if a researcher has several different studies in their "must have/not have participated" lists, they'll have to cross reference the child info across all of those studies to figure out why a particular response session was marked "ineligible_participation".

## Participant POV

"It is confusing when I find a study to participate in, click "Participate now!" to have a quick look and/or decide I want to do it later, and then when I come back I see a red warning about not being eligible."

## Related issues

#1322

Start by reading the issue and related issue #1322, then trace how prior participation produces the "ineligible_participation" status and what consent information is exposed. The issue names no files, tests, or entry points; done would require an agreed design that explains the status without exposing unconsented response details.

Written by the indexing model from the issue text.