build-ui

Legacy feature (has `data/state/` or `services/main-service/`)? Extend it in place per the legacy rules; don't migrate unless asked. Create `ui/`: one folder per UI unit — `<name>/{<name>.ts (lazy wrapper), <name>-element.ts (container), <name>-presentation.ts (pure render), <name>.css.ts}`. The element subscribes to `MainService` state and calls `service.actions.*`; the presentation imports only `data/` and sibling `ui/` lazy wrappers. No business logic here. Comes last. Gate: presentation tests. The how is in the auto-loading `features/ui/index.md` rule (and `element.md`, `lazy-element.md`, `presentation.md`).