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.manager.go

@@ -6,11 +6,11 @@ import (
 
 type Scene = inkwell.Scene
 
-var SceneManager = NewManager(func(entity Scene) string { return entity.Name })
+var SceneManager = inkwell.NewManager[Scene]()
 
 func registerScene() {
 	fillSelectorPins()
-	registerAll(SceneManager, func(entity Scene) {
+	SceneManager.Each(func(entity Scene) {
 		World.G.SceneManager.Register(sceneDefaults(entity))
 	})
 }