Re-sort the current members into an existing Formation

Re-classifies the Audience's current members against the Formation's stored hypothesis and swaps the subgroups in place, without a new preview. Use it after members were added or recalibrated, or to recover a Formation stuck in building or failed. Flips status to building and returns immediately; poll GET /audiences/{audienceId}/formations/{formationId}. Same mutability rules as delete: collaborators for shared Formations, creators for private ones.