Optionalcontext: unknownInvoked once by StuffApi.create/clone after the instance
is registered (so self-resolving lookups during setup see it),
inside the synthetic constructor frame. Witness (async
allowed) — runs class-specific async setup; a throw unregisters
the half-built object. Override and chain super.postRegister()
so mixin layers initialize. Default is a no-op.
Public shape added by PostRegistrationMixin.