Enforces one app-wide GoRouter in lib/routing/ wired via MaterialApp.router, deep-linkable identity in path params never state.extra, context.go-vs-context.push discipline, redirect guards as pure functions driven by a Riverpod refreshListenable, StatefulShellRoute.indexedStack for branch-state-preserving BottomNavigationBar/NavigationRail shells, CustomTransitionPage transitions that respect reduced motion, PopScope (canPop/onPopInvokedWithResult) for unsaved-changes interception, and an errorBuilder 404 route. Use when adding routes, GoRoute, redirect, auth/onboarding gates, deep links, ShellRoute or nested navigation, bottom-nav/rail tab shells, page transitions, back-…
