The control was written for the reflowing PDF reader, but the need is not about PDFs: a long document read as one column has no pages to hold, and "see page 37" is how a room of people refer to one. So it moves out into its own module and the contract becomes one attribute -- an element with `data-page="37"` says page 37 begins here. PowerPoint is where this costs least and is worth most, because a deck needs no reconstruction at all: a slide IS a page, so slide one is page one, and the numbers the control shows are the deck's own rather than anything worked out. One wrinkle worth naming. A PDF's mark is a page's ENDING -- it carries the number of the page that finished there -- so a jump aims past it, at what begins below. A slide is the page itself and is landed on. The mark says which it is (`data-page-ends`) rather than the control guessing from a class name. The arithmetic keeps its tests and gains one: a deck numbers every slide including the first, so stepping back from slide one stays where it is, while in a PDF the same step means the top of the document -- the page above the first mark, which has no mark because nothing ended before it. Signed-off-by: Niclas Overby <niclas@overby.me>