feat(nav): each module declares its URL keys; a page switch clears them and a page opens without another's

The shell cleared a hand-kept key list on a page switch (it missed the Files and feed keys) and cleared
nothing when a page opened from a link carrying another page's keys. ModuleDef.urlKeys now names every
key a page's view writes, the shell derives both from the registry, and moduleUrl.contract.test holds the
declarations true: one owner per key, no shell key, exactly what each codec writes, and no undeclared
key literal in the app.

frontend/src/modules/feed/module.ts

@@ -1,6 +1,7 @@
 import { YouTubeIcon } from "../../lib/brandIcons";
 import { lazyWithPreload, lazyWithPreloadAlongside } from "../../lib/lazyWithPreload";
 import type { ModuleDef } from "../../lib/moduleDef";
+import { FEED_KEYS } from "../../lib/urlState";
 import { FeedStore } from "./FeedStore";
 
 // The rail loads with the page (lazyWithPreloadAlongside), so it never arrives after it.
@@ -15,6 +16,7 @@ export const feedModule: ModuleDef = {
   Rail,
   Page: lazyWithPreloadAlongside(() => import("./FeedPage"), Rail),
   Store: FeedStore,
+  urlKeys: FEED_KEYS,
 };
 
 // The channel page: a feed scoped to one channel, opened over any page (App renders it in place of the