feat: read subscriptions from an Atom feed when a page lists no dated links

SPEC.md

@@ -67,6 +67,7 @@ The v1 client presents one reader screen:
 - A swipe right goes back and a swipe left goes forward through the history stack. The whole empty margin beside the text column accepts the swipe, with a 40 px floor, so the gesture stays clear of selecting text. Touch devices navigate this way; pointer platforms get the address bar's buttons as well.
 - The client's own pages live under the `mews:` scheme. `mews:hub` names them and says what each one holds; `mews:posts`, `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.
+- A site that publishes through an Atom feed instead of dated links is read through the feed (spec 6.2). Dated links are the normative mechanism, so the feed is only asked for when the page itself listed nothing, and a feed on another site is refused. Posts are keyed by address, so a post listed both ways is stored once, under the title the page gave it.
 - Subscribed sites are checked once in the background at launch, and whenever the reader asks. Sites are checked one after another rather than at once, and a site that cannot be reached keeps the posts already held and is still recorded as checked, so one site being down never stops the rest.
 - The reader can tag bookmarks and sites. A tag exists only while something carries it, so adding a tag and making one are the same act; `mews:tags` lists every tag in use and what carries it, and following a tag narrows the list it belongs to. There is no renaming and no empty tag.
 - Each library page carries a search box that narrows it by title or address, and each row carries its own actions: remove a bookmark, leave a site or check it now, forget a page. History lists one row per address, headed by the most recent visit, rather than repeating a page once per visit. Clearing history takes a second step on a page the reader can simply leave, since it cannot be undone.