Drop the duplicate Manager, use inkwell's

The engine already had the registry the game was reimplementing:
inkwell.Manager[T Named], and every entity type here is an alias of an
engine struct, so they all carry GetName() already. manager.manager.go
and manager.interface.go are gone; a category manager is now one call to
inkwell.NewManager.

registerAll goes with it — Each does the hand-off — and the selector
writes its derived pins back with Set, which keeps the scene in place in
the registration order.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

inc/scene.selector.go

@@ -23,12 +23,12 @@ func init() {
 }
 
 func fillSelectorPins() {
-	selector, ok := SceneManager.GetByName(SceneSelector)
+	selector, ok := SceneManager.Get(SceneSelector)
 	if !ok {
 		return
 	}
 	var exits []inkwell.Exit
-	for _, entity := range SceneManager.GetAll() {
+	for _, entity := range SceneManager.All() {
 		if !leadsToSelector(entity) {
 			continue
 		}
@@ -39,7 +39,7 @@ func fillSelectorPins() {
 		})
 	}
 	selector.Exits = exits
-	SceneManager.Register(selector)
+	SceneManager.Set(selector)
 }
 
 func leadsToSelector(s Scene) bool {