CngxAccordionOpenSet

Minimal structural view of the accordion brain the sync drives: a reactive membership read plus the same toggle the native CngxAccordionPanel calls. CngxAccordion satisfies it — isOpen reads the clamped effectiveOpenIds computed and toggle mutates from that clamped view, so single-open arbitration stays authoritative and the exposed openIds model never retains a clamped-out id. The sync writes membership exclusively through toggle, never a raw set-write, so it can never diverge from the brain's own DOM path.