fix(nav): a page settling keeps a reload's overlay entry; an old entry's sub-view goes to its own (review round 4)

- useUrlMirror: a change before the user has done anything since the load is the page settling (the
  rail stepping off a playlist deleted since, Files off a folder gone), not another view: the reload
  mark stays, and the player that reopens takes its entry over — one Back closes it, as before.
- useHistorySubview: an entry an older build wrote is a module's only when its _sub is the view the
  address names under that module's key (the card's push kept the season's `v` beside its `m`).
- Playlists: the rail's default selection and the page's "the rail moved there" read one helper.
- moduleUrl contract: writer literals in a choice's branches are scanned too; the sub-view census
  walks type arguments of any depth and refuses an aliased import.
- A stale comment on the nav press over a live search.

frontend/src/features/navigation/NavigationProvider.tsx

@@ -144,7 +144,7 @@ export function NavigationProvider({ children }: { children: ReactNode }) {
       closeOverlaysThenNavigate(() => {
         // A channel page overlays the content column, so a nav click must close it even when the
         // underlying page is unchanged (e.g. "Feed" while a channel opened from the feed).
-        // A live search on it is one too: the press opens the page's root, as over a channel.
+        // So must a live search on it, though that press ends only the search: the page's view stays.
         const samePage = next === pageRef.current && !channelViewRef.current;
         if (samePage && ytSearchRef.current == null) {
           then?.();