- New app/mobile/engagement/[userId]/page.tsx with fetch + error states + retryNonce
- New EngagementProfileSkeleton with header/metric/breakdown/list skeletons
- 404 renders 'User not found' + back link; 500 renders sonner toast + Retry
- D-04 comment: relies on App Router default scrollRestoration
- D-01/D-22 guard rails: EngagementUserRow.tsx and data endpoint untouched