Function population_view 

pub fn population_view(
    study: &StudyView,
    reports: &[PopulationReport],
) -> Result<StudyView>

Validates a population boundary and adds its reports to a private snapshot.

The caller receives the one shared view every member decision must use; mutations never reach the study’s original view or storage.

Errors

Returns a typed error for an empty batch, inconsistent resource step, wrong objective arity, duplicate/unknown member, identity mismatch, or a member that is not running.