Slot-registration contract a CngxStat exposes to its projected slot
directives. Each slot registers its generated id on init and withdraws it
on destroy; the molecule derives the combined aria-labelledby from the
live registration set. Fronted by a DI token (not the concrete CngxStat
class) so the slots stay decompose-safe - the ejected skin talks to the
same token the library defines.
Withdraw the id contributed by kind. When id is given, the
withdrawal applies only while that id is still the registered one -
a register-before-destroy replacement is left alone.