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>
