- Add useResidentialProjectsFlag hook with NEXT_PUBLIC_FEATURE_RESIDENTIAL_PROJECTS env + URL/localStorage override (mirrors AVM v2 pattern)
- Gate /du-an index (client) and /du-an/[slug] detail (server) routes via notFound() when flag disabled
- Add component tests for index page including disabled-flag notFound branch
Co-Authored-By: Paperclip <noreply@paperclip.ing>