Function enhanceAccordion

Accordion behavior layered on a group of native <details> elements. With allowMultiple: false (the default), opening one panel closes the others. The APG accordion keyboard interaction is added on top of the native disclosure: Up/Down arrows and Home/End move focus between the headers. Items are read live, so <details> added or removed after enhancement take part without re-enhancing. Open state is observable through onValueChange/hl:change and controllable through the returned API; the browser still handles the disclosure widget itself. Markup can set data-hl-allow-multiple and data-hl-default-value on the root.