OptionalgovernsOrgan → vital coupling. A VitalSign key (see lib/vitals/Vitals.ts);
typed string here deliberately to keep BodyPlan free of any
import from lib/vitals.
OptionalinnervatedCanonical dotted path, e.g. 'body.arm.left.hand'.
Tree edge to the parent part, or null for the root.
OptionalseverableCan detach — future part-promotion seam.
OptionalsuppliedNamed tissues + masses — NOT a single default material.
A BodyPlan part merged with this body's instance delta.