Extend lib/modules.ts into an ordered ModuleDef registry (id/label/icon/
system/badge/gate) as the single source for the nav rail and the header
stepper. Absorb NavSidebar's per-page ICON map; derive moduleOrder /
moduleLabelKey / SYSTEM_PAGES from the one array. Render still on the App
ternary — this is the scaffold S6b.3 will drive from. (Platform S6b.1)