List Item At

Returns the source list's element at the (0-based) `itemIndex`, or undefined if the index is out of bounds. Used to access individual positions of a `WindowedMap`'s windowVar list (e.g. window[0] for the first element of the current window) and elsewhere when an author needs positional list access. Out-of-bounds is silent undefined (matches PropertyRead's semantic for missing properties). Authors who want loud failure should validate windowSize / source length elsewhere.