walkElements()function

Walk an Elements value into a flat iterable of Element objects.

• Accepts any shape the Elements union allows: a single element, a (possibly deeply nested) iterable, null, undefined, or a string (strings are skipped — there's no element to yield).
• Recurses through iterable nesting only (e.g. [[a, b], c] flattens to a, b, c); it does NOT descend into an element's own props.children. Walking deeper is the consumer's job.

The point of this helper is to remove the "is it one element, a list, undefined, or some nested thing" branching from every consumer that needs to dispatch over elements — pass it in, get a clean flat iterable out.