The host this update is plugged into, or null when unhosted.
Is this update currently hosted?
Set the back-reference to the host. Call ONLY via the host
chokepoint (AetherMixin.hostUpdate / unhostUpdate); a hard
call-security gate keyed on the calling mixin isn't achievable
(caller-identity resolves to the host's concrete class, never to a
mixin inner class), so consistency comes from the sealed host
chokepoint being the sole legitimate mutator — never from a gate.
Public shape provided by AetherHostedMixin.