Estimate engagement boost reach

Returns provider-confirmed eligible groups and projected reach for a creator at a given tier and engagement type. A recent complete provider roster and exact account-pod membership are required. When matched group member counts are unmeasured, raw_members, unique_reach, reactions, and comments are null with member_data_status=unknown; zero is not substituted for missing audience data. The best-effort audience explanation is directional, not exact targeting. Requires the posts:read scope.