feat: drive the reader with a D-pad

SPEC.md

@@ -82,6 +82,7 @@ The v1 client presents one reader screen:
 - A web page that is not a Mews page is not a broken page. The client reads pages as strict XML (spec 7.1) and the web is not well-formed XML, so such a page is named as the web page it is and offered two ways on: a simplified view, or the reader's own browser. The simplified view is the same markup read as far as the Mews subset reaches — elements Mews supports are shown, the rest are unwrapped and keep their content, and markup that is not content is left out. A form is the one exception to that: Mews has forms, but a web page's form is left out of the view, since the reader cannot usefully answer a search box or a sign-in here and submitting one would only land them on another page this client cannot show as Mews. Only the form goes; content it wrapped is kept. It renders through the same reader presentation as any page, so its links are followed in the app and fail back the same way, its text can be kept as a quotation, and it says it is simplified. Asking for it belongs to the address it was asked for, so a link from it that also fails explains itself rather than simplifying unasked. A page that carries the conformance marker and still will not parse is reported as the markup error it is: the marker, not the MIME type, is what claims a page as Mews, since Mews pages are served as `text/html`.
 - A table cannot hold another (spec 4.2), so a nested one — the old web's way of laying out a page — is shown as a table of its own, after the table whose cell held it, rather than flattened into that cell's text. A row belongs only to the table that owns it, never to one it is nested inside. Rows of unequal length are filled out to the widest, since a table with holes cannot be drawn; a cell spanning several columns sits in the first of them.
 - On pointer hosts the keyboard drives the reader: Ctrl or Cmd+L focuses the address field with its text selected, Ctrl or Cmd+R reloads the page, and the walk goes back and forward with Alt+Left and Alt+Right, or on a Mac with Cmd+Left, Cmd+Right, Cmd+[ and Cmd+]. Touch devices navigate with gestures instead, and none of these shortcuts are offered there. The links on a page are reachable by keyboard as well: the page is one stop in the focus order, and once it holds the focus, Tab highlights its links in reading order and Enter opens the highlighted one; the page scrolls to keep the highlighted link in view, Shift+Tab walks back, Escape puts the highlight away, and a Tab past either end hands the focus back to the chrome.
+- A D-pad drives the reader on any host, so a phone or remote without a touch screen reaches everything. The page takes the focus when it appears, and an error page puts it on its first button. On a page, Up and Down scroll by three lines, and Left and Right walk the links and form controls in reading order, starting from the first one on screen when nothing highlighted is in view. The center opens the highlighted link. Holding Left goes back, holding Right goes forward, and holding the center opens the hub; a press acts when the key is let go, and a hold acts when the key starts to repeat. In a text field the arrows move the cursor and leave the field at the edge of its text, and Up or Down returns to the page. On a touch host the Menu key shows the address bar with the focus in its field and hides it again with the focus back on the page; Down leaves the field for the page, and Left or Right at the edge of its text moves to the bar's buttons.
 - A swipe right goes back and a swipe left goes forward through the history stack. On a pointer host the whole empty margin beside the text column accepts the swipe, with a 40 px floor, so the gesture stays clear of selecting text, and the address bar's buttons work as well. On a touch device, where the column leaves no margin, the page surface itself accepts the swipe: a finger that crosses the slop sideways, before the long press that starts a text selection completes, is swiping, while a finger that held still that long keeps its drag for the selection.
 - The client's own pages live under the `mews:` scheme. `mews:hub` names them and says what each one holds; `mews:posts`, `mews:quotations`, `mews:bookmarks`, `mews:subscriptions`, `mews:history` and `mews:settings` each carry a strip linking to the others. Bare `mews:` is the hub, and the hub is where a session with no history to return to starts. Each page is built as an ordinary Mews document and rendered by the same renderer as a fetched page, so it joins the history stack and the swipe transitions, and none of them is recorded in history or offered for subscription.
 - `mews:posts` is every post from every subscribed site in one list. The reader can order it by date, by site, or unread first, and limit it to unread posts; those choices ride in the address as a query string, so going back undoes one and a swipe preview shows the page the reader will land on. Unread posts are shown bold, and opening a post marks it read.