feat: narrow a library page as the reader types in its search box

SPEC.md

@@ -86,7 +86,7 @@ The v1 client presents one reader screen:
 - 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.
+- Each library page carries a search box that narrows it by title or address as the reader types, a quarter second after they stop. The query rides in the address as any other view state does, but it replaces the step of the walk they are standing on rather than adding one, so a search of eight letters is one step back and not eight; the box keeps what they typed as the page rebuilds under them, and follows the page when the page sets it, as clearing the search does. Submitting the box still works, for a reader who would rather say when to search. 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.
 - A change is a link too: following `mews:do/<verb>` applies it and rerenders the page in place without adding a history entry. Only a link followed from one of the client's own pages is honored, so a fetched page cannot clear history or change a setting on the reader's behalf (spec 8.1). A form on one of these pages is routed the same way rather than sent to the network.
 - Our own pages are built from the same blocks as a page but are interface, not an article, so the renderer lays them out differently: wider before they stop growing, denser, rows divided by a line instead of bulleted, section headings quieter, a note under a row set smaller and dimmer, links carried by weight rather than an underline, and a one-field form on a single line.
 - The interface ships in English, German, Spanish, French, Hebrew, Japanese, Portuguese and Russian. It follows the device's language by default and the reader can choose another on the settings page, where each language is named in its own words so it can be found from an interface they cannot read. A message a language has not translated falls back to English rather than disappearing.