Blueprints declare constructionDurationMinutes instead of tick counts. Delivering the last material starts construction at delivery time via maybeStartConstruction, setting constructionStartedAt/CompletesAt on the site; the game tick only sweeps completed sites. Structure detail page extracts server data helpers and shows a ConstructionPanel with a literal ETA countdown.
Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)
Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>
