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/theme.manager.go

@@ -8,7 +8,7 @@ import (
 
 type Theme = inkwell.Theme
 
-var ThemeManager = NewManager(func(entity Theme) string { return entity.Name })
+var ThemeManager = inkwell.NewManager[Theme]()
 
 const (
 	RealWorld = "realworld-93"
@@ -34,5 +34,5 @@ func RGBA(hex uint32, a uint8) color.Color {
 }
 
 func registerTheme() {
-	registerAll(ThemeManager, World.G.ThemeManager.Register)
+	ThemeManager.Each(World.G.ThemeManager.Register)
 }