Declare the managers in boot.go, hand them to the engine whole

boot.manager.go becomes boot.go and carries all seven managers as one
block, so what the game holds reads in one place instead of a line hidden
in each category file. The category files keep their entity alias.

The hand-off stops copying. Our managers are the same *inkwell.Manager[T]
the Game holds, so New assigns them onto it and engine and game share one
registry per category. content.manager.go is gone with the copy loop.

Themes stay additive — NewGame seeds four presets we would otherwise
throw away — and registerScene becomes prepareScene, which now has to run
before the hand-off: with no copy pass left, the scene defaults are
written back into SceneManager with Set.

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

inc/theme.manager.go

@@ -8,8 +8,6 @@ import (
 
 type Theme = inkwell.Theme
 
-var ThemeManager = inkwell.NewManager[Theme]()
-
 const (
 	RealWorld = "realworld-93"
 	NokiaPunk = "nokia-punk"
@@ -32,7 +30,3 @@ func RGBA(hex uint32, a uint8) color.Color {
 		A: a,
 	}
 }
-
-func registerTheme() {
-	ThemeManager.Each(World.G.ThemeManager.Register)
-}