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/content.manager.go

@@ -1,10 +1,10 @@
 package inc
 
 func registerContent() {
-	registerAll(BackgroundManager, World.G.AssetManager.Register)
-	registerAll(CharacterManager, World.G.CharacterManager.Register)
-	registerAll(ItemManager, World.G.ItemManager.Register)
-	registerAll(DialogManager, World.G.DialogueManager.Register)
-	registerAll(ScriptManager, World.G.ScriptManager.Register)
+	BackgroundManager.Each(World.G.AssetManager.Register)
+	CharacterManager.Each(World.G.CharacterManager.Register)
+	ItemManager.Each(World.G.ItemManager.Register)
+	DialogManager.Each(World.G.DialogueManager.Register)
+	ScriptManager.Each(World.G.ScriptManager.Register)
 	registerScene()
 }