CngxAccordionPanel

Accordion header. Put cngxAccordionPanel on the header <button>; the directive mirrors aria-expanded from the coordinator, points aria-controls at the region, and toggles the panel on click. It registers a header handle with the coordinator on init (and deregisters on destroy), binds its own roving tabindex, and routes keydown through the coordinator's keyboard-nav factory - so arrow/Home/End navigation works even when a skin renders the header inside its own component view (registration is immune to the contentChildren view boundary).

Expansion state is never owned here: aria-expanded is a computed() over the coordinator's open-set, so single-open arbitration is pure derivation, not sibling syncing (Pillar 1). Use a native <button> so Enter / Space activate it through the browser's own click synthesis.