Architect State & Data Fetching: Build custom client-side state architectures and raw data-fetching systems (fetch, AbortController, signal composition, request deduplication, cache freshness, and structured error envelopes with correlation IDs) without relying on heavy external state libraries. Enforce Strict Quality & Accessibility: Write accessible UI patterns (focus trapping, keyboard navigation, aria-invalid, dynamic live regions) and maintain high test coverage using Vitest and React Testing Library.