Which window scenes exist, and which Mob.Router shows each (MOB-245).

iPadOS can show several windows of one app, each a UIWindowScene. All of them run in the one BEAM; each gets its own router, navigation and screen processes. This process keeps the map and owns the lifecycle:

• The primary router is the one the app starts with Mob.Screen.start_root/2. It registers here (register_primary/4), handing over its root module and params, the template every further window starts from. It holds :mob_screen and is never bound to a scene id: it renders to native's default scene, the way a single-window app always has.
• Native reports scenes as {:mob_scene, :connected, id, default?} and {:mob_scene, :disconnected, id}. Native can't message the BEAM before it is up, so this process also asks mob_nif:scenes/0 when it starts.
• A connecting scene that a router already shows is told its window is back. Native's default scene goes to the unbound primary. Otherwise a new router is started for it, bound to its id, from the primary's root module and params.
• A disconnecting scene has its router stopped, unless it was the last attached scene: that router is kept, so the window comes back as it was. When the primary's window goes and others remain, :mob_screen moves to the oldest remaining router before the primary stops.
• A new session replacing the kept one (iPadOS made a new session after the user discarded the last window's) arrives as {:mob_scene, :replaced, old_id, new_id, default?}: native hands the new scene the old one's tap set and model, and the old id's router moves to the new id.
• A kept scene native discards (its session is gone, e.g. removed in the app switcher) arrives as {:mob_scene, :discarded, id}: its router stops, unless it is the app's only one.
• Window events native can attribute to a scene — the back gesture, size class changes, alert results, WebView messages — arrive as {:mob_scene_event, id, message} and are forwarded to that scene's router. One no router shows (a window over native's limit, or one whose router failed to start) is dropped: no other window's router may act on it. Only before the primary has registered does it go to :mob_screen, the one router there can be.

Scene ids are UISceneSession.persistentIdentifier strings, nil on platforms without scenes. Mob.Test.screens/1 reads screens/0. See decisions/2026-10-02-one-router-per-window-scene.md.