Wraps a per-page content function for use in running header/footer regions.

A running-content block carries a function that is evaluated once per page during pagination. The function receives the current page index and total page count as a 2-tuple and returns a list of Rendro.Block.t() to render on that page.

Functions must be pure and terminating. Infinite loops are not defended against (Elixir has no timeout primitive without Task). Use simple, deterministic functions.