Slide shows can carry the whole deck at once

Presenting used to reload the page on every keypress: each arrow re-resolved the next slide on the server and rendered it from scratch, which is slow on a cold deck and simply fails when that round trip breaks.

The slide show now accepts a whole deck up front — every slide already rendered — together with the slide to open on and the shape of the address-bar link. The presenter holds all of them and switches between them in the browser on the usual keys and on click, so advancing is immediate and cannot fail on a lost round trip, while the address bar keeps the current slide's shareable link and Back still leaves the deck in one step. Links inside a slide keep navigating normally, and Escape still exits the presentation.

This is the presenter's side of the change; the deck view starts handing it whole decks next, and until it does, decks present exactly as they do today.