Navigation model

Shared data for all navigation patterns: one source of truth for labels, icons, and routes.

Navigation data lives in a single module so Rail, sidebar TOC, mobile bar, and overlays stay aligned. See src/design-system/lib/navigation.ts.

Each item: id, label, optional icon, destination, group, priority, order, optional children.
Primary items surface in compact patterns; overflow uses overlay / “More”.
Do not redefine labels or routes per pattern — import from the shared model.