An adaptive value whose content is supplied by an external snapshot function, re-read only when invalidated via the handle returned by AVal.ofExternal (FDA AVal.ofExternal parity, MAPA-DESIGN §1.1). Not invalidated → reads are O(1): no re-read, no comparison, no allocation. The invalidate handle is O(1) to call and thread-safe (a foreign-thread call posts to the owner context, the cval.Post pattern); the re-read happens on the next read on the owner thread.