case-route-groups

Cluster: Route Groups (marketing) / (app) with per-group layout.tsx + a group-scoped template.tsx. Route-group folders disappear from the URL, so the two groups share no URL segments.

• About — route group (marketing): uses MKT layout, NO group segment in URL — /about
• Dashboard — route group (app): uses APP layout + template.tsx (TEMPLATE marker) — /dashboard