Structural slot for a CngxAccordionItem's busy state. The item renders
it inside the region while the item's [state] is loading (a first-load
skeleton that replaces the body) or refreshing (a subtle overlay with the
body kept mounted). Absent, the item falls back to its CSS skeleton default.
The region already carries aria-busy as a computed(), so the busy visual
is decorative (aria-hidden); assistive tech learns the busy state from
aria-busy, not from this template. The template receives a
CngxAccordionItemStateContext whose $implicit is the current status
(loading / refreshing), so one override can distinguish the two.