Design the Navigation Architecture

Background: Native [platform] app using [SwiftUI / Compose]. It has
these flows: [list the main flows—e.g., onboarding, tabbed home, detail
drill-downs, modals, deep links].

Use case: Design a navigation architecture that's centralized and
testable rather than scattered across views.

Implementation details:
- iOS: SwiftUI NavigationStack with a typed navigation path, and/or a
  coordinator pattern. Cover programmatic navigation and deep links.
- Android: Jetpack Navigation Compose with a typed nav graph, or a custom
  navigator. Cover back-stack handling and deep links.
- Keep navigation logic out of views; views emit intents, something else
  decides where to go
- Handle: tab roots, push/pop, modal presentation, and deep-link routing
  to an arbitrary screen

Layer & dependencies: Navigation state should be drivable from view
models / state holders, not hard-coded in the view tree.

Deliverable: A navigation design with the route/destination model, how
screens request navigation, how deep links resolve to a destination, and
a worked example of deep-linking into a detail screen.