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/item.manager.go

@@ -6,4 +6,4 @@ import (
 
 type Item = inkwell.Item
 
-var ItemManager = NewManager(func(entity Item) string { return entity.Name })
+var ItemManager = inkwell.NewManager[Item]()